I've never had personal experience in having to do kissing scenes so far, but as the two previous people have said imagine it's someone else. If I have a kissing scene for some unknown reason in the near future I'll just imagine it's my boyfriend and it's one of the greatest kisses we've had to date.
One of the best stage kisses I've seen was in an eighth grade Romeo and Juliet scene and all I remember is that the two "actors" put a sheet of paper between their lips, but made it so you couldn't see the paper and made it look realistic that way. Kind of weird, but that's one of the best I've seen in school.
"The weight of this sad time we must obey, Speak what we feel, not what we ought to say. The oldest hath borne most; we that are young Shall never see so much, nor live so long"-Edgar in King Lear