How much do you want to raise and how far do you plan on going?
I rode across Indiana (158 miles) last summer to raise money for a volunteer trip to the Honduran Foundation for Children with Cancer (I called it the Honduras Endurance Ride...pretty gay, but I liked it). I put the whole thing on myself and although I didn't raise very much money, it was really damn cool to do.
I'm not sure who you plan on asking to donate, but be prepared for lots of rejections. Don't take it personally. I did for a couple things, but it doesn't do any good. Oh, and some people won't come through on some promises, but don't get too upset about that, either.
Just a few ideas: I e-mailed about a million shirt/pen/promo companies and one actually donated about 500 pens to me. A local Lion's Club asked me to come talk to them and I ended up getting $100 out of it (although I was wearing my lucky suit...that suit got me accepted into med school, of all things).
This will sound pathetic, but I think I only ended up making about $300 out of the whole thing. It was an awesome experience though, and if I could go back in time I'd do it all over again. It is tons and tons of work, though. I was constantly obsessing over my website and e-mailing people/companies asking for money or power bars or Gatorade powder or anything.
If you're looking for something easier, you can raise money through a ride that's already organized. This may actually save you money because most long rides have their own support stops and everything like that. I had someone following me in a truck. I forget who the group was, but one group has a bunch of rides across America and they say lots of people on their ride actually pay for the trip through fundraising by asking some charity if they can raise money for them. I'd like to try that either this summer or the summer after that.