I'm trying to keep this spoiler-free. That means I'm not copying my maps and not handholding.

1) Map, map, map. Get some quad-rule and map out every level. Include traps, pools, stairs, and all other features.

2) Good party balance is vital - make sure you have at least one of each magic-user class available, a rogue, and a solid front line (two knights and a rogue or three knights). I have three knights (one of which will eventually become an assassin), a rogue (who, thanks to all the treasure chests, is a good 40,000 EXP ahead of everyone else), a wizard, and a sorceror. Equip your back rank with range weapons asap.

3) Arrange your front rank by vision and dexterity. When handing out armor, give the best armor to the guy in the front.

4) Pools are wonderful for building EXP and getting items - to either equip or sell. Just don't die in a pool.

5) Travelling spells - CANDLE, LOCATION, COMPASS, etc - should be cast as a matter of course. Don't neglect healing between battles - it's tedious, but every HP counts. During a battle, a defensive spell - like SHIELD or WALL - is usually better than a healing spell, especially if multiple characters are hurting. If you have to choose someone to SHIELD first, choose the person with the most (in terms both of spells known and MP left) healing magic left. Draw back towards town if you're low on magic, but the only time you should return to town with healing spells left is a) if you have multiple poisonings that you can't cure or b) if you have levelling to do.

6) Against multiple weak opponents and a few strong ones, use group damage spells to take down the weak opponents instead of letting them nibble away at you while you take down their friends.

7) Be patient about descending lvls. Be level 5 before you try to descend to lvl 2 and be level 7 before you try for lvl 3. My party is levels 9 (most) and one 10 and I'm barely daring lvl 4.

8) Healing potions are more effective than spells.

That's all I can think of at the moment. Have fun! (I know I am!)