We're getting into an area here (pleasing your sub) that spans both male and female Dominants, I think. As Draco has said, communication and knowing the sub goes a long way to helping in the pleasuring aspects, but, too...Dominants take the lead, subs follow. That means the Dominant needs to explore, learn, and practice new methods, techniques, and actions. If we're honest about it, the subs desire to please their Dominant is paramount, so create situations and tasks where their service can do so, and be sure to demonstrate that you (the Dominant) are pleased. That reenforces the sub's sense of self-worth, and the more self-confident the sub is made, the more willing they may be to explore and experiment with "new things."

Another thing to consider is the method of domming. Some people prefer to simply express their desires as "wishes" ('I wish someone would do ________"); others prefer direct commands ("Do this now!"). Find out what method most satisfies your sub's feeling of service to you and use it frequently. Non-verbal commands can be quite satisfying, too. A snap of the Dominant's fingers and pointing, the infamous "look", raising the head and frowning, sure they mean seem melodramatic, but they can -- as ritual -- reenforce the effect of Domination. And, again, being dominated is what pleases the subs.