Dealing with guild hopping

First, you can't fight it. As content deepens, high-end guilds can only recruit geared players, and those only come from other guilds. So the (realistic) question becomes, how to make it work for you, the one losing guild-hoppers.

1) Have a long trial period before people get loot rights in your guild. For a medium-level guild, it's not going to kill you to have people in blues/BOE epics in the raid. And then you've gotten a couple extra months of use out of them before they leave.

2) Give benefits to people who reach milestones. Like, special looting rights or DKP bonuses or whatever at four months, six months, etc. Again, stretch the time they will stay.

3) Get a reputation as a training guild. I know it sounds illogical if your goal is to keep people. But here is the deal. Raiding is an empty, soulless thing, really, and if you make your guild into a fun place where people are mentored and geared and prepared for the next tier, a lot of them will make bonds and stick around. Next thing you know, you ARE the next tier.

Second, if you do want to fight guild hopping any way.

1) Have new members formally agree to "play X amount" and "stick around Y amount," "don't leave when we've got a boss almost down," or "don't quit within X weeks of getting loot," etc. Nothing hard at ALL, just some EASY basic stuff against the most obnoxious guild-hopping. This in itself will NOT keep a lot of people from leaving, sadly; you do it because it is a big help for step 2:

2) Have good relationships with the big guilds on your server, at least with the officers. A good way to do this without it being a political mess or brownnosing or whatever is, for example, when one of your members defects, if you can find reason to, privately and specifically recommend stuff about the player to the guild they applied to. Show that guild that you're fair and unbiased. And THEN, when one of your members really stabs you in the back, 1% on Vael and your MT /gquits, you can calmly go to those big-guild officers who respect you, show them the agreement that that member signed (in step 1) if you have it, or just describe the circumstances, and let them know -- preferably without whining -- that THIS particular person can't be trusted. Some big guilds won't care, but many are looking for excuses to cut apps, and you only have to sink a few defectors before word gets around to "think before you leap."

In summary, I think you're in denial if you think that medium-tier guilds are not stepping stones. However, if you can embrace it and make the best of it, you will collect veterans who will form the core for more, someday.

And keep in mind, when the 25-person instance cap comes with Burning Crusade, a lot of people are suddenly going to be cut loose and looking for stable guilds. Be one!