Further to my post above:
Your campaign design needs to be able to accommodate absent players. IMC absent player = absent or inactive PC, no XP - but PCs below the highest PC level in the group earn x2 XP, so they can always catch up again.
Some adventures are less suited than others to accommodating absent PCs; Castle Greyhawk type megadungeons work better than linear save-the-world plots, but I've used both and you can apply a range of reasons why a PC isn't involved in the adventure. My two favourites are:
They're drunk/hungover/terrible stomach cramps
They're called away on important business (for sandbox games)
When I had an unexpected player absence recently I had her PC shot in the back by a gnoll huntmaster at the start of the session, incapacitating the PC until the player returned. No dice were rolled, let's call it a nice 'dramatic vignette'.
The other players didn't mind since it used up the gnoll's surprise-round action!
I recommend using that only in moderation, though. I've generally found temporary incapacitation through prior injury/disease/alchohol to be the easiest justification. A spellcaster might be engaged in a complex ritual, a priest is praying, a Rogue is ...doing stuff... whatever works for you.
Edit: Oh, one more thing. Stick to the Schedule applies to the GM, too. Just as you still run the game when players are absent, you still run when you have reasons why you'd rather not. With a long term campaign game, a mediocre session is usually better than missing a game.
You have a cold - run the game.
You have a hangover - run the game.
You don't feel very well - run the game.
Your friend is visiting - run the game.
You don't feel adequately prepped - run the game.
You have exams next day - run the game, maybe a shorter session. Do your revision before and after.
You're getting married - you can cancel, the players will understand. Likewise funerals.
I can't emphasise enough how important it is for the GM to be reliable, to set the tone for everyone else. I've seen GMs run with flu, and it wasn't a great game, but it was better than cancellation. If you have really bad flu where you can barely walk you might need to cancel, but that should be the rare exception.
