We deal with this all the time in our group. I have 3 people are solid, hardcore gamers and show up constantly. However, two others are occasional. Here's how I do it. Firstly, no matter what experience I end up awarding them it gets reduced by 75%. I have three basic ways of rewarding characters. 1st: combat; 2nd: Story rewards; and 3rd: Individual Experience Rewards. Combat is fairly simple though I use the FRCS optional way of handing them out if the group has different leveled PCs. Story rewards are from properly completing sections of the campaign/adventures. Individual is broken up into 3 groups (Role-playing, Integrity, and Heroism), rated 1 to 10 and multiplied by 10 and then by level.
Let's say I have 4 people at the last session, all at 10th level. The total XPs for combat and story are 8000. That means 2000 each. Now, one player didn't make it to the last session. So I hand them 2000 too, but not taken from the amount that the other players divided from - just based on it. Once I do that I knock off 25% for not being at the session. So the missing player takes 1500 for not being at the session. The other players take home anywhere from 2300 to 2400 for the session due to not taking the 25% reduction and not getting any Individual XPs.
Here's the thing that will probably surprise you. I don't use the absent player's character all the time. If I have four players, I won't use someone who missed. I will still use the above formula. If I was stern about it and said, "You aren't there then you stood me up and you get nothing!" then the player would probably not show up after their character was sufficiently behind. The biggest tax to those who don't miss is that they come back to the table not knowing everything (though I do send out weekly synopsis' AND they never get the proper amount of treasure. I won't allow the party to split treasure with the missing people unless they find an item they really feel doesn't need sold and would benefit the missing character.
I find this system to make the players who do show up feel better about doing it, though. However, if I ever found out any player said, "I'll just hang out with my buddies and drink because my character is getting XPs anyway!!!" I would be deeply disappointed in the maturity level of said player. The two guys I have not missing are honest with me about why they miss. One has a new girlfriend and they like spending time with her (though he DOES make it if we start 3 hours early which we do often) and the other is a bit burned out right now on regular DnD.
Ok, that was a long post, but it's something I like to think I have experience with as a DM.
Sincerely,
V