Well we raid 4 nights a week, Sunday 6pm - 10pm (allows an early night for monday work) and then Monday/Weds/Thursday 7-11pm, Our guild is mostly 18+ so the school kids that need to go bed early aren't a factor, and the people with jobs can still get a decent 8 hours sleep, while friday/sat are kept free for going out etc.
I don't *require* anything from the guildies, apart from if they sign up to a raid they do so full knowing that it will last 4 hours, maybe you should try some form of sign-ups so you see who is interested in raiding in advance.
However after a few months of raiding we've gotten to the point where we have the same 15-20 core group for 4/4 raids a week, and the rest is made up of people who online / signed-up at the time.
What you need to do is solidify your raid times, find the dedicated people and recruit people to fill in the blanks, assure your key raiders that you will be getting somewhere and ask them to be patient while you build up the raid group.
Don't worry you're in a perfectly normal situation, if you persist you will slowly but surely build up a great raid group.
I'm in a casual raiding guild as well, made up of the over 18 working crowd. We used to raid 4 nights a week but dropped down to 3 due to attendance issues. It seemed to help quite a bit. We raid Monday and Wed 7-10 and Sat 7-11. We hard stop at raid end time. We also have a core group of 15-20 and often have to pick up the last 5 with guildies online at the time. I wish we could have a solid 25 and I curse Blizzard for their bad math! (10+10≠25)