Mike Mearls has a simple solution to the problem with missing players. Run a flashback with the characters of the present players, but at lower level. You might even hand out XP as normal - it's for lower level encounters, so the gap between them and the absent players don't become as wide. And you can provide clues for "current" adventures that are fresh in the players' memories when it becomes relevant, while the clue and the puzzle it solves are spaced far apart in game-time.
In the comments, another Mike provides an alternative - run a bit of the gameworld's history with the players playing important NPC:s. Now you can even kill some characters without weird temporal paradoxes (which you'd get if a character died in a flashback).
And what is it with all these Mikes and Steves in the RPG hobby?