This is probably one of the more difficult questions to answer. Some would like to say no physical contact whatsoever is best while others say that one can do everything up to but not including sex.
In all honesty, both are extremes. It is good and healthy for those who are serious about a relationship to share signs of their love as kisses and hugs. However, it is dangerous and immoral to come so close to sex and to put yourself in a situation where sex is a very real possibility.
Here are some general guidelines:
-Never kiss so much as to “make you want more.” Doing so would be putting yourself in a near occasion of sin, which is harmful and dangerous.
-Never kiss so much as to make yourself forget that you aren’t married. (i.e. remember that you have not become “one flesh.”)
-DO kiss as a **small ** yet meaningful expression of your love. A small expression is all you need at this point in your relationship. (If you disagree with me here, reevaluate your physical relationship.)
-DO share your love as a genuine expression of a loving intention. (i.e. NEVER kiss primarily because you “like the feeling.”)
Specifically:
-No touching. (You know what I mean.)
-No open mouth kissing.
-No passionate kissing.
-Do not kiss laying down. Never lay down together.
I am mostly speaking from experience. However, I strongly recommend reading Theology of the Body for Beginners, by Christopher West and Love and Responsibility, by Pope John Paul II, before he became pope. L&R changed my entire physical relationship, giving me the tools to love my girlfriend more than I ever thought possible with even less kisses.
Please ask if you have any more questions.