Coming from a farm that manages about 20 work traders a year... phew they can be a challenge.
We've tried 2 different setups:
1. Work trade X many hours per season, get your weekly share every week, regardless of how many hours worked.
This worked out well, except that some people wanted to work 8 hour days, 7 days a week to get their required hours in.. others would do nothing up until the end of the season and then try to cram them all in, and others would show up weekly for a few hours every week throughout the season. Honestly, I didn't like this setup because of the infrequency in which people showed up (lots of last minute cancels), or having to tell people "no we don't need you 8 hours today, I really need you 2 hours on harvest day, and 4 hours next week etc etc"...
2. Work trade X many hours per week, and get your share every week if you've worked.
This was my preferred method of dealing w/ work traders. If they were a no show, I didn't have to harvest a share for them, and if they were supposed to be there on harvest day, and didn't show, I'd donate their share to a food bank.
Regarding liability, you should always have liability insurance whenever you're inviting people over to your farm to tour, work, etc. etc. Just never know who will get hurt, or fake hurt, and sue you. It's a tough economy out there and I'd rather not line someone's pockets because I'm too cheap too put out $500/year on insurance. Sure, it's steep but its worth it.
A waiver could help, but I've been told that any lawyer worth his salt can get around those in court pretty easily.
Also, do you realize that if you're giving out more than $500 of produce in barter via work trade that you should still issue a 1099 to them? (e.g. last farm I managed shares were $700).