A few of my clients are in sports and entertainment, so really quick deal memos are what we use. I think that it's imperative to have some sort of agreement with one-off clients; you already know that they are one-off with a potential to grow into recurring - so what's their incentive to pay on time if you don't have a written agreement? You can even have some specific terms and conditions added to the bottom of a written quote and have the client sign off on the quote and agreement at the same time and return to you prior to starting the job.

It also lends credibility to your business when you have an agreement, whether it's a deal memo, letter of engagement, or full blown contract.