I run a game every other week at my place, but we have a dues system that pays for food and props for the group (I'm far too poor to blow my money on miniatures that will only be used for this group). Since we have things like veggie beef stew and fresh baked bread, I need to have things set in advance.
As such, we have a Yahoo Group and the calendar system sends out two notices to everyone for every session. Some of the players have no problem understanding what bi-weekly means, but a couple need to be explicitly told every single week. That's just where their priorities and personal organization is I guess. I urge everyone to post their own times that they cannot game in case we try moving a week because of a majority of no-shows. I have also made it crystal clear when and how I should be contacted in case someone cannot show. Since I did that, it hasn't been a major problem.
Still, some people are just really fricken busy. I've only been able to run my normal game half of the sessions. When we have three or less out of five, we play a fast and loose version of D&D to fill the gaps and toss some dice.