It's all about sticking to your word, once a deal is made, verbally or in writing, stick to it. Until then I practice full disclosure. If I'm working multiple angles, I tell all parties, they are not the only deal in the works and timing is everything. If you make an offer and guy A commits and you reply, "its yours" before guy B chimes in with a better deal, you'd best follow through the deal with guy A.
I'm sure you'll get more advice and possibly ore or better examples. In the end, don't commit to a deal unless you fullly intend to follow through.