If they missed one session, I just have the PC become a "silent" NPC for the night. I will run the PC as DM, or else have a player run him as a second PC, only silent. However, as DM, I'll make sure they don't do anything too out of character - i.e., the wizard charging a hill giant, instead of sitting back and casting fireball at it.
However, several years back, I had guy in a campaign that had been going on for over a year have a job change that would make him temporarily unable to attend for several months. In that case, I had his PC get summoned for urgent family issues (he was from a very minor noble family) and had him "off stage" for the time he was gone. Once the guy worked out his schedule, I worked with him on what happened on his family visit while he was gone, had him level up to be the same level as the party, and then had him swoop in to the rescue against the party. (I worked out the timing with him to arrive mid-session/mid combat and flying on a copper dragon... it was a pretty cool scene with some high-fives exchanged at the table)
While he was "out", though, I had a series of NPCs fill his role as the arcane blaster (it was a big party - this guy's sorcerer, plus a cleric and psion, fighter/paladin, two fighters, a barbarian, rogue/spellthief, and one other I'm forgetting)
So, you could have the PCs in question exit stage right until their schedules work out.