I've handled this multiple ways...
1) If it makes sense, I just have the missing player's character vanish for that session. Works best when folks are in town or near a safe spot. Doesn't work so well in the middle of a dungeon, but it can be used there as well.
2) Run them as an NPC. I really only like doing this if the character is important to the plot or there's no reasonable way to explain them not being there. NPC's gain 1/2 exp.
3) Let another player run them as a pseudo-PC. This is really, really rare, but I have done this at times. This is the method that gets used when a player needs to leave halfway thru a session due to an emergency. They just hand their sheet to another player and let them finish up the session. Since they participated and it's only due to this emergency arising, I don't like to punish them, so they get a full share of exp.
If a PC starts falling behind on exp, dem's da breaks. In my monthly campaign, I have a player that's been a no-show pretty consistently as of late. While the rest of the party is now 9th level, he's back at 6th or 7th. Due to the new exp system in 3.5, if he starts playing regularly again, he'll catch up pretty quickly since lower level characters get more exp. The original 3e rules really punished people and never let them catch up, which was not a good thing. At least with the 3.5 exp rules, there's still hope for them to one day be on equal footing again.