I would be wary of using too large a variety of words (nouns right?) to name body parts. You prolly wouldn't feel the need to go searching through a thesaurus for another name for an elbow or an ear, so why try to come up with a dozen synonyms for a penis? Unless it's done very well it will just distract from the story you're trying to tell.
I believe the best plan is to find one or two words that you're comfortable using (cock, member, whatever) and stick with them. Perhaps you can remove the need altogether by saying something a bit more roundabout, like: 'Hermione took him into her mouth,' or 'Harry grasped Draco firmly in his hand and began to stroke slowly.' Most people will know that Hermione isn't trying to swallow Snape whole and that Harry isn't holding Draco's hand.
The only exception that I can think of would be in dialogue. In that case, think about the character who is speaking and try to imagine what word they would use. Hermione would almost certainly be more technical than Ron would for instance.
Just my .o2