I'd suggest a Bard focus on buffing others first. Stick with a buddy to help him out otherwise. If you stick with the Wizard, you're there to slow someone's advance toward melee with him. If you stick with the Rogue, you're there to set him up for Sneak Attacks by flanking the enemy. In general you can help out by using Aid Another to help ensure a hit from the Fighter (or whatever, there's so much Fighter hate).
In general when using your buffs, try to identify when a fight will be a tough one and use your buffs as soon as possible. If the fight won't be tough, don't use your buffs at all. If you find that your party flees or dies and you still have buffs in your holster, you made a mistake. If you find that you end fights with many rounds left on your active buffs, you should have cast them earlier or not at all.
Finally, as a secondary healer, you can jump in and heal if that's the thing that's absolutely needed. You can spend your round healing 6 damage and keep someone up, but you could have caused 6 damage and dropped an enemy. Try to keep an eye out for that kind of opportunity: if an enemy has been beaten down but it's close to falling, that's the time for you to attack. Same with the friend about to drop: that's the time for you to heal him.
Because you have good movement and low AC, you should probably stick with missile weapons and stay free and back out of melee range. This also helps you rush in and make the necessary move as I described above.