Only a few months ago, (May, to be precise), I performed my first on stage kiss, in the musical, Baby. I had somehow avoided being in shows that required it up until that point. (Though one show in college probably should have included a kiss, I talked my way out of that one.)
My former acting professor once said to his class something alone the lines of "99% of all stage kisses at the amateur level look horrible on stage." I can easily agree.
Yet most people who saw me on stage in May commented on how natural my kiss with my stage wife looked. "How did you keep it from being cheesy", I was asked more than once.
Firstly, I never let it be awkward. I never joked about it with the woman playing my opposite, and we told each other early on that we would do what we had to do, without feeling weird about it. During one early rehearsal, we just said we were going to do it, and we did so. We did not make the mistake of many people in the same position; holding off on it until the last possible rehearsal. Bad idea. This just gives the day when it must occur an ominous feeling usually reserved for an impending tax audit.
Another reason I think it looked so natural was that both me and the person playing opposite me, did not do as many do; simply touch each others lips quickly in hope of it being over soon. Many people hope this passes for a kiss. (It doesn't.) Instead, my co-star and I were aware that the part that sells the realism of a stage kiss is the moment leading up to it, as opposed to the actual "point of contact". If that 2 seconds or so before the actual kiss has feeling to it, what comes after cannot help but look natural.
So if the script calls for a kiss, start doing it very early in the rehearsal process, and do not rush the actual kiss when that part comes up. Replace that awkwardness with a depth of feeling as you approach your cast mate for the kiss, and everything else will usually fall into place.