In terms of character classes I find the BARD a very challenging character to play- here is a character who as the game puts it- jack of all trades, master of none, but as a BARD we do want to have mastery in one area.
Breaking the class down we have thief like ablatives in terms of combat, picking pockets and stealth, mage like abilities in terms of access to lower spell levels, and unique to the class only abilities in performing and singing.
I find that the bard is the ultimate support character, and when played this way not only does it keep the bard out of trouble in combat by not being on the front lines or in a supporting role like a thief, but the bard is what makes the party extremely deadly against Dungeon Master encounters.
At the higher challenge ratings having a bard is a MUST. In terms of spells, forget damage dealing spells- focus on buffs and area effect spells- this frees up spell slots or casts for the wizard/sorcerer who can focus on damage spells, and stacking damage spells to take advantage of their higher level.
At a lower level as an example the bard is better off cast sleep vs. the wizzy. AOE spels like glitter dust also make good choices for the bard.
Buff before an encounter, and during the encounter use use your songs to add more bonuses to the party.
Stay out of combat.