I recently started topping again after a few years of being a bottom only, and it took me awhile before I could cum in a bottom's ass. My dick just wasn't used to it. It took me a couple months of practice before I could finally shoot inside without using my hands at all. Of course, giving up jacking-off 2 to 3 times a day has much has made me a better top too. I also agree that spit makes a great natural lube, and whenever I hook up with a bottom, my first question is always "can I use spit?"