Often a GM or player will want their fellow players to be more interested in the game, to show up regularly and on time, to care if their character lives or dies, and so on. How do you do this? It all boils down to...

Give players a sense of investment

Get players to contribute something to the game group as a whole. There are many ways to do this. These are at the level of player, and character.

Players: The Geek $ Jar

Instead of each person bringing their own munchies, put $5 (or whatever) in a common jar each week, and have one person be responsible for buying munchies with that money. This saves money (as buying for five or so people as a group is cheaper than five or so people individually) and time (as individuals don't have to rush to the store just before the game). Though it's less money than people would spend individually, it feels like more of an investment as the person physically puts money in the jar in front of the whole group. This is especially true if they pay a few weeks in advance. And people always value what they pay for, or feel they're paying for. If you pay for a movie you're more likely to watch it till the end than if you see it free on tv.

An alternate version of this is to have assigned duties to each player. Anna is the one who always brings the drinks, Bob always brings the cheetos, Charlie always picks up the pizza, etc. It makes them more reliable if they know that if they fail to show up, the group will miss out on some particular thing they're responsible for. If nothing else, they're more likely to warn you early before not showing!

Characters: They matter

Create a game world which the PCs can affect by their actions. If their actions will make history, PCs will be much more interested in those actions.

Players & Characters: XP into the game world

Some game systems may allow, or be easily modified to allow, players to spend xp/drama dice/etc on changing something within the game world. For example, in Fate every character has "Aspects" (like virtues and faws, advantages and disadvantages). If the GM charges 5 points for a new PC Aspect, they could perhaps charge 2 points for a new NPC Aspect, so long as that NPC has less than (say) five Aspects. So the players get to flesh out the game world. This gives the players a sense of investment in the game world.

Players: Encourage Journals

When it comes down to it, no amount of xp will turn a person who doesn't like to write into one who does. But those who do write, reward them with xp and (more importantly) praise for their writing. A player who's written 1,000 words about what their character did in session #12, and was rewarded and praised for it, is quite simply more likely to show up for session #13 than one who didn't, or wasn't.

Players: Wikis & Webpages

Those who are somewhat web-savvy can create campaign webpages. If you're not, then use something like this pbwiki. A wiki is a webpage you can edit as easily as a forum post. If you can use forums, you can use a wiki. A wiki is superior to a standard html page for roleplaying campaigns because anyone can edit it. So you can have a page for your campaign world, a page for each player-character - and encourage the players to edit those, put any journals there, etc. For examples, see some campaign wikis here. A campaign webpage also helps you recruit new gamers - they make it look like you've really got your shit together.

Players - Variety is the spice of friendship and gaming

Every fourth session or so, do something different - another rpg, someone else GMs, a board game, movie, soccer game, whatever. An "acquaintance" is someone whom you only do one thing with; beyond that common activity (work, roleplaying, whatever) there is no bond, stop that activity and the acquaintanceship will end. A "friend" is someone whom you do more than one thing with; the relationship extends beyond the common activity, you value them for who they are, not for what they do with you. Quite simply, friends are more reliable than acquaintances, and when they can't make it, you'll be more forgiving, too. Plus, you'll have more friends, which is always nice.

All these are ways to give your players a sense of investment in the group.