There have been probably 100 threads on this topic in the last couple years -
an example: http://www.realjock.com/gayforums/2891195
search on "bottoming" or "anal".
Douche ahead of time - you don't need gallons - a couple ounces, done several times should do.
Try practicing with a dildo or butt plug ahead of time.
Try having him on his back, and sit on his cock - you have a lot more control over how fast things go, and how deep, etc.