Party Etiquette -How to make guests follow rules, without being offensive?

A couple of our neighbors and my family get together and host a large party annually. We include games, inflatable water slide, pool, and ask only that people bring some food or drinks to share, and sign a liability waiver. The waiver states that if any of their family members attending get hurt on any of our properties, we are not responsible for their injuries or medical expenses. We generally have about 100 people at these functions, physical activity, and usually a couple of adults that drink too much. Never have problems, but I don't think that asking someone to sign a paper saying they won't sue if they get hurt on our property, is asking alot. We really enjoy hosting the parties, but it would only take one accident to ruin us all financially. My question is regarding how we can let people know (without being offensive), that they need to sign the waivers or leave the property. I was thinking maybe some signs posted with a funny way to state it, to take some of the sting out of the demand. We know someone who had a dinner guest slip on a wet, tile floor and sue them, so I want to make sure we take precautions. We do want to be welcoming hosts and don't want to threaten our guests with kicking them out. Please provide some suggestions on a friendly or funny way to make our demands clear. Most of our guests are fine with it, but some have refused to sign. So far, we let it slide and pray all night that they don't get hurt. I'm tired of being stressed out about rude people taking everything we've worked for.