What i do is masturbate, wait about 10 mins cuz i really can't be bothered do anything for a bit, then i have a shower for 10 mins on a good heat. As an extra I'll then do a quick few mins with a washcloth soaked in hot water wraped around my penis, doesn't take long after a shower. Worth doing to make sure you're properly warmed up, i don't see masturbation getting circulation going as well as heat would, why not do both I'd say..
Masturbating before is a good way of avoiding getting erections whilst stretching also.