Here's a list of the top five items to include in your terms and conditions. They're simple, and can save you a lot of pain from unrealistic expectations.
1. Sign here
Including a signature page is a good way to formalize an agreement. A signature block can follow a statement as simple as, "By signing this proposal, I acknowledge my acceptance of and agreement with the terms of this proposal and authorize work to begin." If you are providing several options, the signature block is a good place for the client to initial which option they're selecting.
2. This offer good until...
I had once had a client sit on a proposal until the day before an event. Who needs that kind of stress? Now, my proposals begin with, "Please return a signed acceptance no later than January 1, 2017, to secure our services," and the signature block reads "I understand that, due to the nature of the work and the time required for planning and preparation, the terms in this proposal are valid until January 1, 2017. If a signed proposal has not been received by Lizard Brain Solutions by January 1, 2017, Lizard Brain Solutions will release the tentative hold on the event date and may not be available to support my event."
3. Travel costs may change
Another side effect of a client sitting on a proposal means that travel estimates may change. If your proposal includes a travel cost estimate based on rates at the time, be sure to include a plus (or minus) percentage difference that will still be acceptable.
4. Space, time, and light
It seems obvious, but ask for floor space (10' by 5' works for me) to set up your easels or Neuland walls, and access to the location an hour before and an hour after the event to set up and tear down. I once got to an all-day event, only to find out that the organizers set up room changes every hour, with 5 minutes in between for people (and me) to get from one place to the next.
Also under the category of "it seems obvious," make sure there's adequate light. A couple years ago, Tim Hamons was in town to record a huge conference, and he graciously let me shadow him. Unfortunately, the space the client initially provided was in the back of a pitch-black auditorium. You couldn't see your hand in front of your face. It worked out by piping audio into an adjoining room, but it took some scrambling to do it. If there's any doubt of lighting, I pack an LED video light and a light stand with me.
5. This is a partnership!
In the spirit of setting clear expectations, set up the relationship as a partnership. I always end my proposals with, "The more you can engage us as partners in planning and delivering your event, the more we can ensure its success. We look forward to your partnership in this effort!"
Bonus: Have an expert check your terms and conditions
I highly recommend Chris Rader. He's familiar with what we do, easy to work with, and makes it easy for everyone. You can reach him at email@example.com.