I'm not an attorney, and the following is not legal advice. It's just my two cents on this topic.
Recording an Affidavit of Memorandum of Contract will not help you if one has already been recorded before yours. To protect against what you've experienced, Steve, I'd simply do the following.
1) Get your contract signed and executed between you and the seller.
2) Confirm with the seller that they haven't signed any other contracts to sell the house and that they won't sign any other contract to sell the house, because the contract they just signed with you is legally binding and must be the only one.
Because the appraisal districts often take a while to get documents online that have been officially recorded, even an online search for other Affidavits of Memorandum of Contract for the house you're buying may not show recently filed docs even though they've been filed before you got there.
One more thing... A common problem is that sellers think they have the right to rescind a contract they've signed within the last three days. It's my understanding that when that law does apply that it only protects buyers, and that, when it comes to real estate transactions, it doesn't actually apply to either party.