Here are some things I wish I knew before starting.
As everyone else is saying, save often. My process went like this. I would enter a new map and make a hard save. Then I would quicksave frequently as I traveled across the map. Once I resolved an enemy encounter, I'd fight, sleep, save, then continue.
There is no "mana" system for spells. You only get so many spells per day. Once you use a spell, you can't use it again until after you sleep.
Spell levels don't coordinate with player levels. For instance a cleric can't use 2nd level spells until he reaches cleric level 3. He can't use third level spells until he is a level 5. He can't use 4th level spells until he's level 7.
I discovered a technique that I found out later is called "Kiting" or "Pulling". All my characters had at least one proficiency point in some kind of ranged weapon (bow, crossbow, or sling). I'd scout ahead with a fast character and arrange the rest in a semi circle. Once I found a monster, he would chase me. I'd lead him back (like a kite) to where the rest of my party could ambush him with arrows, bolts and bullets. When he changes which character to chase, simply move the new one. This is a good way to build up early levels. It only works when the enemies are few and there is a lot of room to move about.
EARLY ON FREEBIES! These items are very easy to miss. They can all be acquired at level one. Enemies will stand in your way, but you can take them out or avoid them if necessary. Skip if you want to find them yourself, but I don't see a reason to:
On the path to the Friendly Arm Inn, just north of where you meet Xzar and Montaron is a skinny tree with an expensive diamond to sell.
Follow the path to the next map to the east, just a little over a screen length north & east of the monument at the start is a rock with a +1 Ring of Protection.
Bottom Edge of the middle of Friendly Arm map is a rock with a Ring of Wizardry which gives your mage more first level spells.
Left side Nashkel town in a tree in the fields is +2 Ankheg armor.
If you're playing on Normal difficulty, change it to Easy before leveling a character to maximize their hit point roll. Some may consider it cheating, but it sucks to see your hit points rise by only a point or two. Don't forget to change it back or you'll get less experience on future encounters.
Find and set the AI for your characters. "Records" (R) screen, "Customize" button. Read through them. I set everyone for either ranged or melee. Sometimes I will set the specialist ones (offensive cleric, defensive druid, etc) to see how the computer will handle those situations.
In combat, concentrate your attacks on one enemy, usually. I set my pauses to "friendly took damage" and "enemy dies". Once it pauses I decide to use a heal spell/potion or change the focus of my attacks to the next enemy.