This is a wide topic and I do hope that by summarizing the tips, it'll help owners with dogs who are prone to allergies.
If you suspect your dog has allergies to something,
1. Visit the Vet. Let's face it, visiting the vet is the first thing you should do. Rather than trying to figure what allergen triggers your dog's reaction by yourself, have the vet check him out first.
2. Corticosteroids, anti-histamines and other medications. Your vet will probably give you information on how and when to administer these meds, don't forget to follow instructions.
3. Allergens.
Food. In order to narrow out the possibilities, food is the first thing owners often change in a dog's diet.
Environment. That could include a lot of things, from the detergent you use to the pollen count in the area. Even the plants you have in your backyard could be the culprit.
Parasites. Some dogs may have severe allergic reaction to tick or flea bites
4. Omega 3, herbal and natural remedies and holistic vets. If your vet is not able to pin point the problem, you might want to consider getting a second opinion- may it be from another medical or holistic vet.