Difficult to know exactly what you're classing as 'nice'.
I tend to flip between being submissive and wanting my five minutes of control. If it isn't something you're comfortable with keep it pretty simple, 'force' him (give him a bit of a push, lol) onto his back, if he tries to do anything with his hands slap them away/gently hold them down on the bed, do a bit of biting etc.
You've just got to think what it is you like about being submissive and put that onto him at a lower lower, initially at least. Start small, you'll probably find the small amount of inner dominance that you have will come out and play....if it doesn't then you've learnt something further about what you like sex-wise!
you could tie him to the bed (using soft scarfs or something of the sort), blindfold him and then tease and pleasure him for as long as you want before actually getting on top and riding him, this way there is no force used and it is a very mild sort of dominance.
I don't think you can dominate a guy in a nice way. Just as I hate to be dominated by a man, even if they try and do it in a nice way, you can only repress someone's soul/spirit for so long, before the bolt!