one word, or should I say one acronym: PUG.
Yeah, it's not the ideal solution, but this problem you have isn't just yours. It's a problem for many people as summer rolls around. Plus this time it's even more difficult with people 'retiring' until cataclysm.
Odds are you're gonna have to PUG it. If you badger your raiders too much about it, you'll only anger them and you run the risk of loosing them completely as opposed to just loosing them for the summer, only to get them back in full force with a vengeance after summer and cataclysm ships. Instead of doing that, the only sensible option really is to PUG. And PUG intelligently.
What's Pugging intelligently? It's the art of networking, which you should probably be doing anyway if you've got a larger guild. As more people hit 80 and gear up, more will want to raid, and to save yourself the strain of setting up additional groups and whatnot, networking with other guilds to take your guildies over random pug members from trade pays off.
Same basic concept here. Want a 25-man but can only muster 12 people? Say you've networked with some other GMs and Officers. You check with them, see if they wanna come and goof off and, hey! look at that! they've got 13 people who wanna roll a 25!
12+13=25 gloriously geared raiders ready to roll. Problem solved.
Now admittedly, you might not have those connections right now. You might not WANT to have them. Well, that's okay. Because Pugging is still viable. there's still lots of people out there who wanna get their 25-man done. The difference is that when you team up with another guild, managing a multi-night 25 can be done easier.
Sure, you can try tricks and bribes to get your missing raiders back, but that risks alienating them, which presumably you don't want. If they're not showing up to your raids, then clearly they're not passionate enough about it that they'd be alienated by you pugging their spot. and if they are, explain the nature of the situation patiently and remind them nicely that if they want to keep their spot, they need to be showing up. You can't get loot or down bosses if you're not there. This isn't Basketball. the 13th man doesn't count for squat. We don't raid before a live audience.
"I don't have a dream, so I protect the dreams of others."~Inui Takumi