As someone who waxes for a living, I'll throw my 2 cents in.
I don't ever draw a line on anyone, nor were we taught that in school nor have I ever met anyone who does. its a good idea, and maybe you should draw the line yourself to let them know what stays and what goes.
Make sure they trim the length of your eyebrows before waxing. Some people have long hair on their eyebrows, and trimming it down by first brushing it up with a disp. mascara wand and then trimming with tiny scissors works really well.
To help numb, you may try some anbesol applied and then washed off before your appointment to take the bite out of waxing. Its just lidocaine, so it temporarily numbs the skin.
I would make sure your waxer uses pot wax as opposed to the roll on ones, as you don't have as much control with a roller and the quality of wax thats in those rollers isn't great.
Again, have a good consultation before and make clear what you want. Also, if you are having problems growing them in, try a product by a haircare line called Nioxin called Follicle Booster. Search on google or yahoo, or check a local salon to find their products. ULTA stores carries it I know.
Don't tweeze or pluck at home for 3-4 weeks for optimal growth for your first wax. Also, don't try to "maintain" tweeze. It just isn't worth it. I can't even do my own eyebrows, so that says something right there. Leave it to a professional. You need that objective person who is more cautious and has that outside perspective to work on you.