Good for you for getting the work done, but educate yourself a bit too before you go in. Drink a decent amount of water before you go in and a TON after the massage.
Deep Tissue can and release a lot of toxins into your system. If you are not properly hydrated your lymphatic system will not properly flush this crud out of your system. And you will be twice as sore and feel flu like symptoms.
Do not, I repeat, DO NOT be shy to tell the therapist that they are too deep or are using too much pressure. If you bruise the muscle too badly, this is counter productive and will only slow down your therapy. Don't get me wrong, you will probaly be bruised here and there, but you don't want to feel like you were hit by a truck the next day.
And finally, stretch pretty good for a couple of days. Before and after the massage.
Other than that, good luck with your massage and I hope that it's an enjoyable experience for you.
JM