My girlfriend and I have also been running our own guild for just over a year. (
Crimson :: Horde :: Gurubashi). We have run into similar problems but have had a fair bit of success in keeping raiding going for the past 14 months.
I had a couple questions and maybe some suggestions.
How many days a week do you raid?
Are the bulk of the people who leave the guild just quiting the game? or moving on to another guild?
How many "core raider" positions do you have?
When you are not recruiting, how many people are sitting out every raid?
My guild runs a 3 day raid schedule. Because we only raid 3 days and want to clear all content we expect everyone to make all 3 days. We run with a raiding roster of 28-29 raiders. This means that only 3-4 people can miss a given raid which does happen during some holidays but we have a good selections of "friends/members" to choose from on most nights.
My suggestions without reading any answers to the above questions would be to keep recruiting until you have 27-28 people showing up to every raid. This way, if you have 1 or 2 people leave your raids don't stop all together and cause a chain effect or gquits.
I'm sure most guilds have had similar problems this summer. We decided to take a summer break of 1months, to give longtime members a chance to do other things. It's hard to expect people not in a top 5 guild to raid constantly for years on end.