First option: The "pink bubble". If you miss a session, you are protected from any harm, but also gain no benefit. Your character wanders around with the rest of the group (so no risk of being left behind if we teleport to another world or something), but can't contribute in any way (ie: spells, skills, knowledge, etc). There's no risk of your character being killed, but you also gain no skill/XP/etc.
The pink bubble works well for any skill-based system because there's no obvious comparisons between characters, and you aren't dealing with key abilities being gated behind level accumulation. If you didn't get a chance to level up your sword skill this week, oh well. There's no sense of "falling behind", unless you miss a ton of sessions. It didn't really transition to level-based systems like D&D, but still works if you don't want to let someone else play your character.
Second option: Allow another player to play your character. This is what we do with most level-based systems, including D&D, as long as you're only going to miss the occasional session. This means you gain XP with the rest of the group, and the group gets access to your spells/skills/knowledge, but your character is subject to the risk of injury or death or loss of items, or whatever else might happen. In fact, there's probably greater risk, because other players won't be as well-versed in your abilities as you are.
This option assumes you can trust your friends to play your character fairly. It comes with the assumption that the DM won't allow any abnormal behavior, such as looting your character, or using him as a bomb detector, or other obnoxious things.
Third option: Set up a side adventure for your character with the DM. You lost your chance to be part of the "main" adventure (which may involve more loot and/or XP), but you can still have fun doing something, and it often allows small quest hooks to be dropped in that can be used with the main group later. Rather than "no XP", this is more of "less XP" if you get explicit XP rewards. If you do milestone leveling, then you should still be keeping up with the rest of the group.
Extended leave: Character is removed from the roster and does not advance any further, or on a slightly slower track (eg: 1-2 levels behind the main party). Is not available as a playable character for the rest of the party (as in option 2). Don't have a lot to say on this option, as usually it meant a permanent removal of the player, or by the time they got back we were on a different game, so they'd be making a new character anyway. Occurs when someone's work schedule changes, or someone in the military goes on a 6-month tour, etc.
I don't like the concepts of punishing the player by withholding XP (even if couched in terms of "encouraging" the player to not miss sessions) because it reminds me too much of my MMO days, where attendance became a club to beat people with, and it almost guaranteed that it was eating away at real life because you were punished in a myriad of ways for not being in game. When I see those types of arguments, I think, "You're the type of person who ran a guild and made my life miserable."