Learning that you or a loved one has a food allergy does not mean the end of going out to eat.  In fact most restaurants are familiar with preparing foods for those who have allergies and those that are not can soon learn how.
For your part the first thing you need to do is learn as much as you can about your particular allergy, there are many foods that may be part of the allergy that you would not necessarily think of.  For example if you suffer from a peanut allergy you should also avoid foods that contain lecithin and certain food additives.
Of course since restaurants serve a variety of foods the possibility exists for cross contamination even if you are very careful about the dishes you order.  For this reason it would pay to download or order copies of the Food Allergy Restaurant Card that can be found online at several locations.  This card explains to the wait staff and the cooks that you have a particular food allergy on the front side and that is can be a serious medical problem.  It also explains what they should do in case of an allergic reaction including calling for medical assistance.
The back side of your card will list the particular foods that you are allergic to, including the need for using dishes and utensils that are new and not cross contaminated.  Most restaurants are well trained in helping with food allergies and will not have a problem with this, if the one you choose has a problem then it might be best to go elsewhere.
Another thing you can do to make your meal more enjoyable is to look at the menu before you go out; you can do this by dropping by on a day when you are not planning to eat.  You can also call ahead to ask the restaurant to have food that is made for you, in fact most restaurants would prefer that you do it this way.  There is nothing worse for them than to be caught off guard in the middle of the dinner rush.
Trying to dine out when the restaurants are not quite as busy is also a good idea since this will allow the chefs to work more carefully on your order. The most important thing is to be very intent on making sure that the staff understands your allergies.  You need to let them know before you order; it is too late once your food has been delivered to tell them you have an allergy to something on your plate.
The key to dining out safely is no different than eating at home you need to educate not only yourself but those around you.   Once you have found a restaurant that will cater to your needs be sure to patronize them on a regular basis and tip well.  Remember it takes a lot of extra work to cater to the needs of someone with allergies and not all restaurants may be willing or able to do this.