Choose higher quality restaurants.
"Nicer" restaurants will be more likely to have the resources-and possibly the know-how-to accommodate your dietary needs. They often create their menu items from scratch and, therefore, can tell you the full list of ingredients in the meal.  At more "fast food" type places, many foods have been pre-cooked elsewhere, and so the staff may not know what was done to the food prior to it arriving at the restaurant. I have found this to be true for some meats and sauces, especially.

Stick with a few restaurants to dine at regularly.
The restaurants you are loyal to will be loyal to you! Not only will they begin to understand your needs more thoroughly, but some may even offer to stock products for you (a brown rice pasta, rice milk, etc).

Call in advance.
Let the restaurant know about your needs and ask them if they will be able to accommodate you. It is better to know in advance, than at 7pm when you are already starving! Additionally, the restaurant may want to prepare things and set them aside in advance-before the dinner crowd hits!

Know the right questions to ask. 
Start by saying "I have a serious allergy to "X". Do you know what that means?"

If they answer yes, ask them what they understand. You will usually know very quickly if they REALLY understand what you mean!

If they answer no, are unsure, or are inaccurate, then you will need to go into more detail, ask for a different waiter, speak to a manager or chef, or provide them with an allergy card (see below).

On my recent trip to Florida, I had one waiter who I felt was not taking my gluten allergy seriously. He was practically walking away from the table, as I was still explaining what I couldn't have. And, sure enough, two-bites into the halibut that he told me was only crusted with macadamia nuts, I knew there was breadcrumbs!

Allergy cards.
Allergy cards provide food-allergic people with an easy way to notify restaurant staff about what they can/cannot eat. You can visit this website and create your personalized allergy-card. When traveling out of town, print off several so you don't have to worry about getting your card back.

Use the Internet.
If going on extended travel, do some internet searches in advance. Look for the nearest health food store so you know you'll be able to pick up necessary food items when you arrive. Also, simple GOOGLE searches such as " eating gluten free in Las Vegas" can provide you with a good start for restaurants you may want to contact.  

Also, chain restaurants such as PF Changs, Carabbas Italian Grill and Outback Steakhouse are providing GF menus for their clientele, so these are great places to dine when traveling to a new place.