By and large, I agree with Randofu. You overthink these things, they go bad in a hurry. However, I've got some minor points to add to the first writeup..

as to the preparation: The word you're looking for is water. Water, as well as being refeshing and a boon to one's health, makes for far more pleasant kissing. Dehydration of the mouth can result in a particular sort of nastiness marked by the congealing of saliva. Drinking water will make your mouth feel friendlier. Odors (an unfortunate side-effect of the necessary act of eating) will dissipate more quickly, little particles will be washed away, and the no-rock'n'roll-fun sticky spittle will cease to be a concern.

Stay away, on the other hand, from the sweet stuff. Sweet stuff = sticky mouth, and I feel we've covered the merits of that oral disposition pretty thouroughly at this point. Worried about your breath after a meal? Eat some bread. Bread likes to absorb things. A nice, respectable bread you have to tear at will also help get things out from between your teeth.

The bottom line, though, is that you have to determine how much you really want to be kissing whomever you intend to kiss. If it's someone you like a lot, hopefully they like you, too, and will be too caught up in the actual kissing to be overly concerned with the smell or taste of your mouth and all other matters involving this earthly plane. On the other hand, if it's someone you're just making out with for the hell of it, who gives a damn how your breath smells? Have you ever seen "For make-out sessions flavoured with a vaugely detectable aroma of macaroni & cheese, call xxx-xxxx" scrawled on a bathroom wall? No, me neither.

as to the setting: If you've ended up kissing someone, it's a moderately safe bet that your setting is just fine. If you're trying to seduce someone, it might be worth your time to worry about mood lighting and whatnot, but the biggest variable determining whether you'll be getting smooched or not is your own charm and the quality and tenor of the conversation.

Not a lot can stop a good kiss. You forgot to lock the door and your roommate walks in mid-tonsil inspection? Provided you're not doing that silly thing where you pretend you didn't know you were kissing each other, laugh it off and resume. As an added bonus, you now get to go through the whole sexy eye-contact-placing-hands-bumping-noses thing again.

as to the breathing: Breathe through your nose, sillypants. If you think this might end up being a problem, blow your nose soundly at some point earlier in the evening.

as to the actual kissing: Relax. Don't rush it, don't try and second guess the other person or act coy. Don't do anything you're uncomfortable with. Do whatever you feel the inclination to, but being ready to respect the other person's wishes, should they ask you to stop. The nice thing about sexual matters is that your body knows what to do in these situations. It doesn't need the assistance of your mind, though it's certainly nicer when the kissing isn't completely mechanical. Things will sort themselves out. Have faith.

..and good luck!