I think that kissing is sort of like crying( read that great thread)...if you get the physical appearance of the kiss right - position of hands, tilt of the head, the pause, the swoop, the long look ...then you don't really have to be mashing mouths together.  Some contact -  yes - but not as much as in a real kiss.  Maybe your actors would be comfortable with just a slight pressure of lips. 

On the other hand I don't think that a forthright mention of mouthwash and breathmints is out of line.  Everyone has bad breath when you're not in love with them.

I avoid, side step or cut the mushy bits out in a production. I believe they are not necessary & if they are relevant to the script, I always try to leave it to the punters imagination. The ones at the end of a scene are easy - fade to blackout, before impact!
The odd one that can?t be avoided are handled as Wolfman suggests.
My old mate, who is a fellow techie, has always complained whenever he gets to play a part, that he never gets to snog the bird on stage - He has finally got his wish! He is playing Emile de Becque in South Pathetic & he gets to snog Ens. Nellie Forbush.
He rang me up & asked the question of how he could  avoid the kissy bit, I advised him that I had no sympathy for him & laughed saying I could not see any way around it at all!  His only possible chance to get out of it & avoid the ?Bird Flu?, was by pulling the pin & just doing the lighting!
That will teach him for having a fantastic Welsh baritone voice & being at the theatre during  audition time!
[Half his luck as she actually is a gorgeous young girl who is playing Ens Forbush!]

I'm sure I've addressed this issue on other posts,
but BOTH actors need to understand that a staged
kiss is no more than a set of blocking used by the
playwright or the director to convey their message.

Oddly enough, it should be treated the same as a
staged fight. Trust needs to be established,
boundaries set, and the action choreographed to
avoid injury or embarrassment.

Whenever I direct a scene like this, I always defer to
the Female Actor and her sense of boundaries or
propriety. If either actor is reluctant or uncomfortable,
I try to assuade that issue by asking them, "How do
you think the CHARACTER would behave in this
situation?"

This gives them the opportunity to say something
like, "Well SHE (or HE) would probably .... [fill in the
blank]."

Whatever choice they make, it becomes understood
that it is the CHARACTERS that are kissing, not the
actors. The actors are mere vessels which the
characters inhabit.

And -- above all -- do NOT send your actors off to
rehearse the kissing scenes "on their own." Your
presence ensures everything remains businesslike.

Personally, I don't believe in "stage kisses." There are enough elements onstage that you have to fake by necessity, and a kiss isn't one of them. During the casting process, I'll always ask those auditioning whether they'd have a problem kissing, so that when we get into production, they have been duly warned.

 

The most difficult kissing scene I had to assemble involved two teenagers, who broke into giggles at the very thought of having to kiss onstage. I remedied that by holding a special rehearsal closed to everyone by myself, the two actors, and my AD. I made them run through the two kissing scenes dozens of times, until the novelty was gone. It took a while for the rest of the cast to get used to seeing the scene without snickering, but the key was getting the two principles comfortable. Everything fell into place after that.

I second the bit about the stage fighting metaphor -- the other thing to do, besides setting boundaries and all, is to talk about the purpose of the kiss.  Just like a fight or a line, a kiss is communicating something.  Is the kiss about true love?  Is it about power?  Ownership?  Nervousness?  Is one character in love but the other trying to make a friend jealous?  If you talk about what the kiss is trying to accomplish, I think the rehearsal will go much more smoothly.  Going through the dramatic purpose of the kiss sets it up as something driven by characters,a nd will probably give you the type of kiss that you're looking for.