I'm a newbie and don't know what is usual and customary in tipping a provider.  Should tipping be done before or after a sesion.  Please help...

Count me in too. I need the same help with tipping protocol and social graces.

Keep in mind that a waitress makes close to minimum wage and makes up by the subsidy of her tips

A provider IS the restaurant (and a pricey one at that)
If you feel her service has been exemplary and/or wish to win her favor for a future tryst; have at it. Otherwise; she has been well compensated already just by the donation.  

If you want to tip then it will be appreciated but it's by no means necesary.

Personally, I believe tipping should happen after a booking and only if you've had a great time and you want to make that clear to the provider.

I use the same philosophy as with restaurants and cabs; about 15-20% depending on the circumstances.  When a provider allows me to stay a significant amount of time over, I also take that into account according to their rates.

What is the protocol? Leave it discretely?  Whip out the wallet and pull out the bills? Slip it to her secretly on the way out? Is there a best practice.  Also, can the tip be something other than cold, hard cash? Could it be a gift card to a favorite store, etc?

Keep in mind that only about 40-60% goes to the provider for non-independent incall - so even a relatively small tip goes a long way!

during the goodbye hug, I slip my hand into my pocket and grab the bills and then grab her hand and discreetly place them in her palm.

Many providers suggest gift cards in the FAQ's on their sites and if this seems like a good idea, go for it.

I would think a gift card is appropriate for a tip after you have seen the lady at least once.  If it's the first time and you don't feel like tipping after all, then you're left with a gift card you might not easily find a use for.
If it's and incall, I generally leave the tip the nightstand or dresser as I'm getting dressed.
For an outcall, I typically leave it on the dresser when she is in the bathroom.
In both cases, I direct her attention to it by saying that there's something for her on the...
No money changing hands may be a little overboard at the end of the session, but I feel it's respectful.