#1 No Lubrication
Just because you're mentally ready doesn't mean your Private part is. Studies have shown, it takes 5 to 7 minutes for a private part to be sufficiently lubricated for fun. Also, some things have the tendency to dry out your private part - warm showers, allergy pills, and some birth controls, to name a few. Be sure always have some lube by your bedside! You might not always need it, but it's a lifesaver when the mood is right.