Completely relax your lips. Don't even worry about puckering. Go slow!!! Don't be nervous because of that sexual tension you have right before kissing. Build it, keep going in slowly... build it... then SLOWLY brush your lips against hers... n slide in for the lip lock.
When you rush things, you could catch her lips in a bad position - Possibly where you are eating her lips. When you take it very slow... you can even open your mouth pretty wide on the first kiss, she will feel this and adjust. (Because lots of girls give you a pucker type of kiss on the first, no lip lock)
You can even go for the french if you wanted too. Key is to take it very slow, allowing her to feel what's going on and adjust.
Once you get good at this. You can do more advance kisses. Ones that you don't often see, and involves head movement. Check out Trey Songz at 3:36.
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http://www.youtube.com/watch?v=E98IYokujSY[/youtube]