I've heard lots of success and horror stories out there, and as a single guy who loves being that extra "spice" in the bedroom, I figured I'd add a bit of my experience and knowledge to the forum. By no means do I consider myself an expert, but I am a communicator and I've heard the good and the bad (and the ugly) when it comes to men trying to play with hotwives/couples.

1. Communication is key. You're entering an established relationship. Make sure you understand what that dynamic is. Rules, limits, turn ons, all of it. Know your boundaries. Even I tried to kiss a wife during play and was stopped politely as they don't kiss members of the opposite sex when they play. Sometimes it's a deal breaker for folks. But communicate, be open to hearing them.
2. Treat them like they want to be treated. If it's a situation where he likes being talked down to or humiliated, make sure you're comfortable doing that. If she wants to be center of attention, make her center of attention. It's pretty simple, common sense stuff, but I've heard of guys completely throwing caution to the wind when it comes down to play time.
3. If it's their first time, make sure you take it as slow or as fast as they want it. Maybe they want some leadership or advice, if so give it to them. Or they want to make sure this is the right situation for them. Being a sounding board helps sometimes too.
4. If they wife asks you to play alone, be sure it's good with all parties. I had a situation once where I got invited to play with the wife, and she assured me he knew. He came home early from work...clearly didn't know, and it was like a scene from a movie where I had to bounce out of the house through a window. Very embarrassing. So cover your own tracks too, as your safety and comfort level is also important.
5. Get. Tested. Some couples play safe only, others don't, this is a no-brainer but take some time to get tested regularly so you're more confident in your play.

That's all I've got for now, for anyone in the lifestyle, what else would you want to add? Do you agree with what I'm saying here, am I off base?