Best advice is listen.......

Listen to CDs, and learn the parts off them....

Listen to other, local bands and watch what the keyboard player does

Listen to the rest of your bandmates, and don't step on what they play

Listen to board tapes of your rehearsals, figure out what works, what doesn't

Listen to a teacher, it'll be the best money you ever spend (if they play rock!)

Listen, even (especially!) when you think you already know enough!