Usually we go with juggling: cutting back and forth between the solo PC and the others.
It's not a perfect solution, because sometimes you can't make quick cuts and that leaves either the solo PC's player bored or the rest of the group bored, but them's the breaks. We've talked about it in our group and eventually came to the conclusion that with five players to deal with, everyone has to accept that they won't be in the spotlight all the time, and sometimes it's actually necessary for one character to go off and do something alone. Everyone tries to be courteous about it.
That said, some games and some GMs have managed it better than others. We're somewhere below average on D&D in general (higher full-group participation for hack-and-slash, lower for intrigue-based games), above average for the Marvel Universe RPG (something about the ruleset encourages more participation, maybe?), solidly average for Farscape d20, above average on Feng Shui and most other one-shots or pickup games, and totally abysmal at Vampire.
Actually, the almost ridiculously lopsided cuts in the Vampire game (where we have had several sessions in which one particular PC has not actually been able to do ANYTHING) has set off a whispering campaign among our group's other potential GMs. Our agreed-upon goal: every game session, every character gets to do something, and ideally just as much something as any other character got to do. With special attention paid to the poor player who has been the most consistently sidelined in the Vampire game, of course, because we feel so bad about what she's had to put up with.
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and so the great wheel keeps turning
ryan