HINT SUMMARY

GENERAL STRATEGIES

ITEM STATS: In general, try to figure out if something is better than what you've got. For instance, a stone spear has stats of 2-20 -1, whereas an iron spear has stats of 2-20. The stone spear has an average damage rating of 10, but the iron spear has an average damage rating of 11. Go for iron. This is pretty much always true. The hierarchy of items is bronze/crude/poor/stone < iron < steel < blessed. Look in the equipment section for a stat summary.

FINDING STUFF: Look in items. Press "L" and choose something. The best stuff is found in crates, barrels, dressers, bookcases, desks, and of course chests. On rare occasions, you can find stuff in other objects- for instance, in Fort Ganrick, there are some casks in the equipment room which have some cheap wine hidden in them, which you can take.

STEALING: It ain't a crime if you don't get caught. Many items have a "NY" next to them. If you try to take the item, and someone is watching, you get in trouble (and may even instantly die). There are ways around this.

FOOD: There's a big selection of food in this game. Generally, I go for the lightest (greens), and ignore the heavier lizard haunches and fine meals. If you want variety, pick up all that you can. You should never need to buy food, as there's always plenty lying around.

LOOT: From luthien on the spiderweb message boards: If you're loading up on loot and hit the max items, you can pick up a couple more things if you equip the non-magic jewelry (or any other items like bows and missiles that you don't already have equipped). This is typically only useful earlier in the game before your characters are completely decked out. Also, you may find yourself unable to carry any more.
There are a few ways around this, from Zeviz: There is also a (slightly cheating) way to carry much more than your weight allowance. When you are trying to equip an item, it does not go through the weight check. So if you equip that super heavy Empire armor straight from the ground, you'll be able to carry it even if it takes you over your weight allowance.
And from TIE187: Also, if your party has a strength bracelet, you can pass it around and equip it on each character when they're picking up loot. It adds 60 stones to the character's carry capacity, up to 350 stones. Again, "the problem is that you can't drop anything since you won't be able to pick them up again."

QUESTS: Quests are your main source of reputation, and a good source of gold and xp. Whenever you complete a quest, be sure to get your reward! Uncompleted (ones where you've not received a reward) quests can be found under "Current quests" in the character info screen.
From luthien on the spiderweb message boards: It appears that the penalty for experience is random. If your penalty is -50%, I think the 50% is applied by giving each point a 50% chance of occurring. Either that or some other distribution. I've completed quests worth 30 XP where the 50% penalty character will get anywhere from 10 to 20 XP. The moral of the story is, save before you complete a quest or some other big-XP action. It will be hard to get max XP for all your characters, but at least you can prevent one PC from getting cheated too much.

WANDERING ENCOUNTERS: Whenever presented with the option to be friendly, do so. More often than not the others will react with friendliness or indifference. If nothing comes of this, save, then attack.
From luthien on the spiderweb message boards: Since loot and experience are so valuable, I will usually save the game right before any wandering encounter. Then reload the game to make sure I get the maximum number of enemies. 8 bladesman, 6 empire archers is better than 5 and 3...

LEARNING CRYSTALS: You'll occasionally find one of the really nice learning crystals. They're small blue things that come in three flavors- abilities, skills, and energy pulse. Ability crystals give you a new ability, like lay on hands or something. Skill crystals teach you 3 points in a given skill (for some skills, hold on using these until you have purchased as much skill as possible to get the maximum result). Energy pulse crystals just dumbfound you, but are good to sell at 599c apiece.

MAGIC BARRIERS: There are three types: fire, which can be walked through for damage, force, which are impenetrable but can be removed with a dispel barrier spell, and Big and Weird, which are those which are separating Avernum and which you'll run into again later.

HERBS: There are two types of herb patches- one is found on the outdoor map as a special encounter, the other is found in an actual dungeon. The latter are much better as they have more herbs than the outdoors ones. There are 3 dungeon herb patches. Come back to either of them every 5-7 days or so to pick up some more herbs.

SINGLE PLAYER AREAS: There are a few occasions in which you have to select a specific character to do a quest. This is one of them. In general, choose the character best suited for the job (obviously).
From TIE187 on the spiderweb message boards: For single PC quests, I use my Mage/priest. I give him or her all the protection and special armor the others wear: shielding spike, 2 or 3 amulets and jewelry, rogue leather, lockpicks. Having your choice of Priest or Mage spells and the large amount of manna that goes along with training in both is quite nice.

NPCS: All NPCs are stronger than your characters at their respective levels, for the most part. There are a few role playing considerations, but if you want a powerful party, get them to join you. NPCs also get character traits based on their position of entry: if they start as the first character, they get elite warrior, and as the last, they get natural mage.

SPELL CASTING: The power of one's spells are based partly on luck, and partly on the mysterious Bonus. The mage bonus is equal to the mage spells skill + int/2, and the same is respectively true for priest spells. There are some ways of increasing this ability through wearing items, acquiring skills, or choosing specific character traits. The mage's bracelet and the robe of the magi both give a +4 bonus to your character. However, wearing both does not give 10 bonus, so it is best just to wear one. The magery skill gives a bonus equal to the number of points in this skill. The natural mage trait gives a bonus of 4 at the beginning, and may increase with levels. For priests, there is only the priest's bracelet. The mage's bracelet and the robe of the magi do not affect priest spell strength, however, both magery and natural mage do.
More on Natural mage from Alex: Unlock Doors level 1 is the easiest way to see how the bonus is calculated. My result:
The bonus is incremented every 7th level, so the benefit of Natural Mage is:
+4 at level 1
+5 at level 7
+6 at level 14 etc.
There is also an increase of 0.4 per level whether you have Natural Mage or not (i.e. it's on top of the Natural Mage bonus). This makes Divinely Touched (+1 regardless of level) look even less attractive.

TO HIT: Interestingly, there is no 95% cap to fighting unarmed. Even so, the damage done is pitiful, so I wouldn't recommend this.

COMBAT VS MONSTERS

DEMONS, DRAKES: At low levels, you can take them on one at a time with the bind foe 1 spell. Cast it once a turn, and pummel them with all of your other characters. At high levels, they should present too much a problem, just wait with your mage till the end of the turn, and cast haste to void their slow spells.

WIZARDS, RAKHASHI: Only try these guys on higher levels. They can easily destroy an unprepared low level party. The bind foe 1 strategy can take them out, one at a time, but it's more time consuming. A decent strategy is to summon up a null bug, and either sit in the antimagic field, or get the bug right up next to them, and hit them with your weapons. Missile weapons are quite effective on these guys too. Cloud of blades is the only spell that can hurt the rakhashi.

DOOMGUARDS: Hit hard and fast. Divine warrior, blessed and hasted warriors are necessary. Do not use the jade halberd while fighting these guys. Monster summoning is also generally a bad idea, since they are immune to magic, and will split even if they take only one damage.

DERVISHES: Eventually these guys are only a mild nuisance. Still, at low levels, bind foe 3 works wonders. Most damaging spells work too slow to be of much use.

LICHES, HAAKAI: Pray they don't hit you with their charming ray- it's quite difficult to resist. Otherwise, the bind foe 1 trick could work quite well, and repel spirit is rather powerful.

GARZAHD: Demon slaying arrows are the way to go, honest. They do 50 damage a hit, the best possible. Don't worry about his non-spellcasting friends; they actually will help you by diverting his arcane shields.

GENERAL: Haste works wonders. Before entering a battle, cast haste (or beast ceremony), then (w)ait, being sure that the enemy doesn't do anything. This'll give you an extra attack.
Some hints from Zeviz on the spiderweb message boards: If you open a door to find a room full of hard monsters, you can close the door back by clicking look button and looking at the door. (Warning: this does NOT work in combat mode) Now you can proceed with advice 2 and 3.
2. If you see monsters in a dungeon, but they haven't seen you yet, you can still cast Beast Ceremony if you close a door in front of you or run behind a corner (or wait until your light runs out).
3. If you are in a dungeon, faced with the enemies who have no long range attacks, you can cast your defensive and summoning spells without entering combat mode. Since the enemies move 1 square per turn when not in combat, you will have more time to cast spells before enemies cover the distance between you and them.