Czae and Joph pretty much hit all the biggies. All classes require some experience in order to play them "well", but the bard class is one of special note. Many people don't really understand what a bard can bring to a group, they just know they want one. That's because having a good bard in your group can make it
seem like the group is much more powerful then it really is. If you pay attention, you can fill the role of ALMOST anybody in the group that may go down for whatever reason. There will be many times you'll save the group, but not necesarrily by tradional means. If the tank goes down, you probably won't be able to jump in and tank in his place, but you will be able to keep it mezzed till the tank gets rezzed and rebuffed, or kite it around, or maybe drag it to the other side of the zone so the rest of the group can zone out, or highsun (gate it, as Joph said) it to buy everyone some extra time... the possibilites are almost limitless.
In order to excel as a bard, the most important thing you can do is LEARN what your songs do. Don't just have a vague idea. KNOW them. After your 30's you start to get a lot of "combo" songs that do more then one thing. KNOW which songs stack so you can get the best bang for your buck. KNOW exactly what each song does so you don't get undesireable side effects. Sure, Song of Midnight is an AE fear, but it sucks when you realize that you forgot that it also SoWs the mobs as they're feared and puts a minor DoT on them as well making them unmezzable. KNOW YOUR SONGS.
Other then that, one of the things I like to do when I'm grouped is "just have fun". I know that's always the goal, but it can mean something completely different for a bard. People get nervous when I get bored. I start pulling 3 or 4 mobs at a time, seeing how well I can time the mez breaks to get them staggered and all into camp at various intervals. Or, if they're charmable, I'll charm kite while people are AFK. Or I'll play hide and seek with mobs just to see how they react to being pulled (that actually helps when you go to pull them for real). Most of the time, you won't have time to play around a lot because you'll be so busy with your songs and pulling, but when you have a solid group... why not
?