As an actor, you'll have to perform all sorts of different scenes throughout your career. Kissing scenes are simply part of the job. Though they can be awkward initially, there are things you can do to make the process more comfortable and the end result realistic.
Read the entire scene thoroughly. You need to know where you are supposed to be when the kiss starts, throughout the kiss and when it ends.
Discuss the scene with your partner. If you feel awkward about the scene, tell him. It's likely that he feels the same way, and it may help to get it out in the open.
Remember you are both professionals. The kissing scene is just another part of the job.
Talk to the director about the logistics of the scene. Kissing scenes can be very technical. You have to be aware of the camera at all times and make sure you aren't blocking any shots.
Plan the exact blocking of the scene with the director and your scene partner. This will help you feel comfortable when you shoot the scene.
Use breath freshener, breath minds or mouthwash prior to rehearing and filming the scene. It's professional courtesy to avoid bad breath before a kissing scene.
Relax. Try to be as natural and realistic as possible during the scene.
Avoid using your tongue in your kissing scene unless otherwise specified by the director. Most often, kissing scenes can appear to be more passionate than they are due to camera angles and acting ability. Only perform kissing scenes in a safe environment. If you are at all uncomfortable with your director, scene partner or any aspect of the production, excuse yourself and contact your agent or manager. If you do not have representation, voice your concerns to a member of the crew or the director.