Contracts often include a notice to terminate. You want to negotiate for as much notice as possible in the event that the client leaves. You can also ask for "the right of negotiation" or the "right of first refusal," which gives you a chance to throw in a bid if the client wants to go to a competitor.
Having a big brand on your client list isn't as valuable if you can't tell anyone about it. Terralever asks each client if it is willing to be used as a case study on Terralever's website and whether Terralever can use the client's logo on marketing materials.
If your company will be creating something -- say, a new technology -- for a client, ask for shared rights to the intellectual property. COO Scott Miraglia usually asks for shared ownership of the software code Terralever produces for its clients' interactive marketing campaigns. "It can become tricky," he says. "We want to take the code and repurpose parts of it for other clients. They want to be able to use it anyplace, anytime."