Here's some free advice from a law student - don't rely on free legal advice!
Liability waivers are necessary, but you need to know what they cover and the extent to which you are covered. e.g. - if you are found to be grossly negligent, the court will nail you, regardless of what the other guy signed.
The laws are different state by state - using a form you pulled off the internet which might be perfectly valid in California might not protect you at all in New York.
I would recommend paying for a local lawyer's advice - it could save you a ton of money down the road.