book keeping

One of the most important things about DnD is continuity, especially in a larger campaign that stretches over months or even years. In cases like this, especially at the age after you leave high school (something thats well past for both me and DK) where people often have many commitments and DnD can be sidelined, leaving the party down a member or two for any number of sessions. While it can be easy to adjudicate easier encounters for fewer people, this is rarely the problem.
The problem is maintaining the continuity.
If there is anywhere over a week between sessions it can be difficult to remember exactly what went on, leaving people in the dark over important encounters, meetings and NPCs. Plus all the great random jokes and weird comments which are always a big part of our campaigns. This is an especially big problem if, for whatever reason, there is a dry spell in your sessions.
The easiest way to prevent this is to keep a diary of events as to what's going on each session. Unfortunately this will put even more pressure on the DM.... unless one of the players keeps it. This is easily the best way to handle the situation.
A player picks up on all the things that they enjoyed most from the session, giving a different light on the encounters and the PCs. you know who they hate, like, what they enjoy, what they skip over as unimportant. Plus it maintains the ever so important continuity and makes a great way to ease into the session at the beginning.
The problem may be finding a willing player, but lets face it, they owe you a little something for preparing that great session they're having.
As a side note, how on earth do you only spend 1-4 hours creating a dungeon? that DMG for 4th ed suggests this is entirely possible. I sure couldn't.