The three that may've helped me most:
1. Don't neglect fortification. I remember debating with myself if I should use a +1 Con and Heavy Fortification item over a +4 Con with Light Fort. Switching to Heavy Fort was one of the best things I did for survivability.
2. Constitution shouldn't be ignored. Figured that one out quickly with a 10 Con Ranger I made once.
3. Dual wielding weapons/using weapons which you're not proficient as a caster is fine, especially if you're not planning on hitting stuff in melee. I foolishly thought the penalty for dual-wielding hurt my wizard in other ways or something.
4. Learning how to properly select/target and inspect both enemies and friends alike. Simple enough, but I remember not bothering/thinking to inspect anything for the longest time. Doing so has saved me much time and resources while educating me some and making the game a little more enjoyable.