Well, it can be difficult to make the jump into 25 man raiding because attendance is usually poor when people think a scheduled run isn't likely to have enough people. It's a self-fulfilling prophecy.
I've found that the best cure for this is to make sure that you are raiding consistently. The easiest way to do this in a guild who doesn't have attendance is to team up with another guild in a similar predicament. You may have to try a couple different guilds before you find one that meshes with your guild's personality, but once you start getting 25 mans happening regularly, people will be more likely to log in. It also makes recruiting easier. The other advantage to running with another guild is that they likely already have a reasonable set of tanks/healers that you can draw on.
I've found that without a sudden massive infusion of new players in a guild which is struggling with attendance issues, usually the war of attrition wins. You may recruit a couple people, but then a couple people leave the guild, or start a new job and can't log on, or quit the game, or start a project overseas or whatever. It ends up being a revolving door with no real gains or losses. Getting a big infusion of new blood, whether it's just in joint runs between separate guild, or whether it's being absorbed or absorbing another guild, stimulates the guild. It gets people motivated again. On top of that, once you have a reputation for being a 25 man guild, and you start downing bosses, more people start to apply, and it becomes a lot easier to maintain that attendance.
That's advice based on my own guild, which has gone through any number of difficult phases, but always come back stronger than ever.