It's not that there is one particular site. It's what Sun&Moon and others talk about in their post responses. It's quite simply knowing yourselves, being honest about what you want and getting to know people. I sometimes use the term "vetting" someone - it sounds a bit business like but that's kinda what you need to do IMHO.
It often goes sometihing like this for us - we find a profile which is well written and intelligent with some pictures that actually look like some time and thought was given to selecting the pictures. The Mrs has her type and she will say let's see how it goes. The initial conversations are with me and the Mrs reads them later (she will join the conversations when she feels like it). A good number of the men usually flake at this point or some flags show up that have us thank them and we move on. If the conversations go well we meet in a public place for a drink and see how we all get along. There is often a difference between an online personality and a RL personality. If that meeting goes well we will play. The only time we have ever met someone and played that same night was with couples. We never have done that with a SM.
So really try and get in tune with what you guys are looking for and take your time selecting someone to be with. My wife calls me the PR person, she doesn't get interested in all the talking and sorting through the messages, I actually enjoy it. To me it is very much like dating in that you will talk to a lot of people and eventually end up meeting the right one at the right time and those end up usually being the best dates for us (both 3 somes and 4 somes).
When we first started we didn't understand what some folks meant when they mentioned they wanted to "click" or have the "right vibe" with people. We do now. It's a slower process but we aren't looking for numbers and so far we have not had anyone stand us up and no show on a meet. Even the few folks who after we have met and did not end up playing are still friends.
Take the time to understand what you both want out of it. Understand that, at least for us, we are not looking for someone to date or such, just someone who we sexually click with and it all just kinda flows when it's right.