It doesn't hurt to have directions written down on a piece of paper that you can send to the kitchen with the pasta, such as when you know you prefer to have the pasta cooked for 8 minutes, or if you like a splash of olive oil in the water while it's cooking. Also, be sure to remind them to cook your pasta in a clean pot, drain it using a clean strainer, and not to stir it with a contaminated utensil.
Depending on how well you know the restaurant, it's sometimes better to call ahead and mention you'll be doing this (and if they tell you you can't, you can cancel your reservation and go somewhere else).
Most chefs are thrilled to help out in this way, however. I've had chefs and restaurant owners come out and write down the name of the pasta brand, so they can start to stock it for other gluten-free customers.
