One thing you should NOT do is ramming and hoping the best. But surely, you wouldn't. With my girlfriend, it's always very tight at first, but once she's reached her climax once she's relaxed. So I give her head at first, or (when it has other tastes/smells than the usual, nice ones) use my fingers until she comes. She then tells me to grab a condom (which since the first few times has worked as an intense turn-on for me). We are going to experiment with lubricants shortly, so if you want I can tell you what I've found out. She probably want me to give her head anyway though..
You also might look into lubricating gels made for sexual activity.