If you lease a car, borrow money for school or engage in some other complex transaction, there's a contract to sign. It's filled with rules and obligations, and the profit-maximizing finance organization does everything it can to do as little as it can (and make you responsible for as much as it can). This sort of contract has evolved into a battle, an effort to get something now and deliver as little as possible later. Loopholes and fine print are there for a reason, and it's not to make you happy. Contracts like this are about the past. "We agreed on this, go read your copy, we don't care so much that you're annoyed, goodbye."
A handshake deal, on the other hand, is about the future. Either side can claim loopholes or wriggle out of a commitment, but the consequence is clear—if you disappoint us, we won't be back for more. The participant in a handshake deal is investing in the future, doing more now in exchange for the benefits that trust and delight and consistency bring going forward.
It might pay to write your handshake deal down, to memorialize the key promises in an email. If your goal is to delight and to exceed expectations, the more clear you are about the expectations, the more likely it is you'll exceed them.
But it also pays to hesitate when you (or your advisors) start pushing to transform the handshake about the future into a contract about the past. "I'm hoping to do this again with you," evokes a very different reaction than, "but you said…"