Kick Out Clause - gives the seller the most protection and lowers the risk of losing out on a potential sale. If you get a better offer, you can give the buyer a set amount of time (e.g., 72 hours) to remove the sale contingency, or the deal is off. The current offer is "kicked out" and no longer valid after the specified time period.
Backups - with this agreement, the seller is free to accept offers on a backup basis. Potential buyers can check out your home and make an offer on the property even though you accepted a contingency offer. Backup offers are contingent upon the original buyers not meeting their purchase agreement obligations.
Less Time - if you feel the buyer has asked for too much time (e.g., 90 days) you may demand a 60- or 30-day contingency period.
In Escrow - the buyers' home must be already sold (and in escrow) before you will accept the offer.
Deposit Forfeiture - potential buyers forfeit their deposit money if they cannot close on the house by the specified date. Although many buyers may not accept these terms, the seller has the right to include them. This removes some of the risks from the seller and places them on the buyer.
Refusal - you have the right to refuse contingency offers, but by doing so you might limit the amount of purchase offers you receive, and it could be harder to sell your home.