I find it hard to get a man off in cowgirl - and, yes, that's how most of them start out. I don't mind starting out that way as it seems natural to do that after the oral/putting on the condom.
To change positions I will say, "Wanna fuck me in doggy while we watch ourselves in the mirror?" (I have a very good, large, strategically placed mirror.) Even though they sometimes say, "Not yet - I want to do missionary" it still sounds like a sexy suggestion. Which hopefully disguises my true motive (to get them out of my face and get them off quicker.)