Use Index Cards for Reminders
How many times did you forget to add in a temporary modifier? How about that feat which gives you a bonus to hit or a modifier to your armor class? Or that bonus you get from something that another member of your party did (e.g. the bless spell)?
To keep from missing out on these create a reminder card for each. Take an index card and write down what the skill/feat/spell is along with the highlights and then you can place the card in front of you while playing.
For example, I’m currently playing in a 3.5 D&D game running a Duskblade and one of the spells he has at his disposal is, magic weapon. The card created would have Magic Weapon across the top and would state the +1 enchantment bonus granted on attack and damage rolls along with the duration, 1 minute/level.
When the spell is cast during the game the card can then be placed in front of me as a reminder that I need to add that additional bonus.
This works for the GM as well. Be sure to create cards for the same types of items your players will along with reminders for spell like powers of the various creatures the party will encounter and the secondary effects of spells, traps and poisons.
May your dice roll well.