“Aww man, I got stuck playing the Cleric again…”
“Just be quiet until we need healing would you?”

Has this ever happened to you? Maybe you wanted to play a fighter, but the group really, really needed a healer. Sometimes a rogue (thief) is needed for their unique expertise. All is not lost. Don't throw your character on his sword just yet.
Just because you’re there to help other PC’s doesn’t mean your character has to be boring.

There are lots of ways to spice up your support character:

“Look what I built- it has three attack modes and can do your taxes.” Don’t be afraid to explore the limits of your character’s abilities during the down times. Work with the DM and make exciting new inventions (or research new spells.) Hey, even if they don’t work, you’re sure to draw attention.

Track down tangents in the story line that others may not have picked up on. Or, you may just have grabbed a hint the DM didn’t plan for. Either way, run with it.
Get involved with the group. Get to know the people your character is working with. If nothing else it will give you something to talk about while you’re patching things back together.

Speak up! Don’t be a one line wonder. “Ya, I heal you…” Sometimes you might even want to talk to the henchmen NPC’s if things get too quiet.
Prevent having more work for yourself through the use of etiquette and diplomacy. “You know, we could just try talking to them, better yet, send the Shield Guardian…” Maybe you’ve got a skill or idea that will prevent casualties.
Do not fear combat. Your character might not be that bad at it. And it’ll give you a chance to try out new toys, spells, saving throws or whatever that you don’t get to use at any other time.

Use your unique abilities to your advantage. (But don’t abuse this.) “Ya, I’ll heal you, but you must first promise to introduce me to the king’s daughter.” (Just don’t go overboard) “I’ll bring you back from the dead if you’ll sign a contract that I get first pick of the magic items we find for the next 10 levels.”

Background, background, background! The more in depth your character is, the more you and the DM have to work with. Just because you’re in the rear with the gear right now, doesn’t mean you weren’t someone with an exciting life five minutes ago. It gives the DM more avenues for your character (and the group to explore.) You don’t have to write a novel, just a few interesting tidbits.

Do some preemptive work for other PC’s. Cast bless. Loan out your huge-o-matic crossbow that you built out of spare parts. Chances are, you'll have more time to prepare for upcoming events than other characters. Clerics and Druids tend to be able to cast spells in a preemptive fashion, anyway. It's one of their class specialties.
Last but not least, get involved in the plot. You’re the one with all the Charisma, why should the drooling basket case with the sword speak for the party?