Be freshly showered (meaning you had a shower/bath just prior to the meeting, not a day ago).

Fresh breath, brush teeth just before the lady arrives, or even while she is there, or even both.
Good idea to brush your tongue too, IMO.

Be well groomed. No long sharp nails. No rough stubble. Hairless balls will be appreciated, and be sure to clean all those naughty bits that often get missed, such as ass crack and foreskin. Especially if your uncut.

Have the envelope with the gift in placed where the lady can see it easily, and absolutely no talk about money.

Be respectful at all times, and be respectful of her time.

Always leave a tip, unless you are unhappy with the service.
After all you don't go to a nice restaurant and not leave a tip, do you? Well you shouldn't...LOL.

You will already know what the lady offers having visited her website and read her reviews.

But if there is something your not sure about, don't be afraid to ask.
For example I like a bit of dirty talk during a session. But only if the lady is happy to go along with it.
So I will always ask if she is ok with that, and if so, are there any words she would not be ok with.

I am here to have a good time, and not upset or annoy my companion.
If she is happy, I am happy and we will have a great time together.

I know the above list is pretty obvious, but you would be surprised how many guys don't do the obvious.

Good Luck, and Enjoy your first experience

You're about to spend time with a gorgeous lady with no strings attached.
What could be better than that?

Seeing as you did your research and chose a reputable lady, you'll be fine.
J_UK gave you excellent pointers.

When the date is over, send her an email or text thanking her for her time.
Showing your gratitude will go a long, long way.

If there is anything that would make your experience more special let her know. She may have an entire arsenal of tricks that you will never know about unless you request them. With a newbie I usually stay pretty vanilla unless he lets me know he wants some adventure.  
Good luck!
Heather

Check out my review list...

They all fit your bill.  If you have questions about any of them PM me and I will be happy to help.  Trust me, whatever you're looking for, it's there somewhere... assuming they are still in business...  

Thank you all for the advice and helpful links. I'll take your counsel to heart.