My first method is group checks. The traditional example is a difficult bit of travel: if you're going through a desert, maybe you call for a group Endurance check, or if you're climbing a cliff, maybe you call for a group Athletics check. In skill challenges, group checks require half the group or more to succeed, so for a five person group three of them would need to succeed.
If you do this, I recommend setting the difficulty level of the check at easy by default, because some characters aren't going to be trained in whatever skill you're using. That doesn't mean you can't use harder difficulty levels, of course -- just think about it carefully.
A lot of published skill challenges try to make this work in a diplomatic setting. E.g., the King says he wants to hear arguments from everyone before he'll agree to whatever it is that needs doing, or the madcap faeries want everyone to entertain them. In my experience, these often wind up feeling artificial. If you want to do something like that, foreshadow it rather than just dropping the group requirement on the players out of the blue. Perhaps people mention that the King tends to be concerned with the kingdom as a whole, not just individuals.
My second trick is specific skills. You can identify key skills that only the quieter characters have, and make a skill challenge that calls for those skills. This works better if it's always specific players who aren't participating, of course. I'm usually willing to drop some hints here, or even just call for a character to roll. For example, say there's some nature-oriented skill challenge going on, and the quiet guy has Nature, but he isn't coming up with any ideas. I'll just ask him for a Nature roll and if he succeeds, I'll feed him some information.
That works much better for knowledge-oriented tasks, since that way you're just telling the character what he knows rather than trying to dictate his actions. On the other hand, you can turn almost anything into a knowledge-oriented task. "Hey, Bob, roll Streetwise... OK! You've always been in tune with the streets, and while everyone else has been running around you've noticed there're a lot of thugs drinking at that White Heron bar over there."
You can make that kind of forced roll give a +2 to future skill checks rather than counting as a success or a failure, btw; that also reduces the feel of railroading. You're not walking them through the skill challenge, you're making sure they're able to apply their knowledge well.
My third method is splitting the party. If there's a good reason for the party to split up, the guy who always does all the Athletics checks might not be there when he's needed. A sneaky little variation of this that I've always wanted to try: capture the party spotlight guy -- you know the one, the guy who's always up front and pushing the ideas and so on -- and let the others do a skill challenge to break him out. The focus of the game is on him, which should satisfy that urge, but everyone else gets to be the active participants.
Finally, as always, talk to the players. I don't like to kill the flow of the game by over-analysis during play, but I find it's useful to chat a bit afterwards when I'm working on something like this. It's always possible the players are looking for something in skill challenges that you aren't giving them, and if there's one person holding down most of the spotlight, maybe he doesn't realize it.