As a relative newbie, I am aware that the donation is to be placed in an unsealed envelope, etc., and left discretely on the nightstand/desk upon arriving at a provider's room.  Also, that no discussion of rates relative to services is appropriate.  That being said, what is the proper etiquette should I decide the session deserved a tip, or, for instance, if the provider and I mutually agree to extend the session.  The original envelope has disappeared.  What do I do with the additional cash??

after you do the deed and you want to give more, just hand her the cash or place it on the nightstand.

Some guys bring a second envelope for that.  Just be discreet and don't make a big show of counting or saying how much it is.

During the goodby hug and kiss, I will have the bills in my palm.  (Keep the tip in your pants pocket so you don't have to pull out your wallet.)  As I pull away, I take her hand and discretely transfer the bills into her palm while saying nothing and keeping eye contact.

It's a nice, romantic and tender way of handling an otherwise awkward mission.

Thanks for the advice Bengal and mrfisher.

By the way, shortly after I posted this question, I noticed there is an interesting thread on the General Discussion board regarding tipping.