In the spring and fall many people suffer from seasonal allergies caused by a reaction to the pollen from trees, grasses and weeds. Frequently symptoms include nasal congestion, runny nose, and watery itchy eyes. Other symptoms include headache, lethargy, skin rashes and asthma. There is more information about allergies at About.com. 
The most common conventional treatment for allergic rhinitis is the use of antihistamines. They work by blocking histamine which is released by the body in response to the allergen. The first generation antihistamines are quite effective but they are sedating (ex. Benadryl or diphenhydramine). The second generation antihistamines have fewer side effects and are thought to be equally effective (ex. Claritin or loratadine). Nasal corticosteroid sprays are now used as the first line treatment for moderate to severe rhinitis because they are more effective than antihistamines (ex. Flonase or fluticasone propionate). Cromolyn is an over the counter allergy preventive that stabilizes the mast cells. It works by reducing local inflammation in the nasal tissues. Finally, allergic desensitization (allergy shots) is still used in patients with severe allergies but always carries the (low) risk of life threatening reactions.

There are many different integrative approaches to treating allergies. These treatments have been poorly studied but in most cases carry fewer side effects and they promote healthy immune function rather than blocking it. Here are some ideas:
Botanicals
There are two primary herbs that are used to treat allergies; butterbur and nettles. Butterbur (Petasites hybridus) is better known for its use in the prevention of migraines but in one short term study with 132 people it was found to be as effective as Zyrtec and to have fewer side effects. Stinging nettles (Urtica dioica) has a long history of use for the treatment of allergies. In one randomized double blind study 57% of the patients had relief of the nasal symptoms.  
Nasal Irrigation
Keeping the nose free of allergens is another important approach to the treatment of allergies. Even Kaiser recommends nasal irrigation. They suggest using a bulb syringe. A gentler and easier way to irrigate is to use a netti pot. You add a little sea salt or kosher salt (non-iodized salt) and warm water and pour it into one nostril and out the other. Here is a video of how to use a netti pot from YouTube.

Nutrition
A common recommendation among integrative providers is to follow a low protein diet as proteins especially from milk and animals can irritate the immune system. Obtain protein from plant sources. Maintain good hydration and increase your intake of bioflavanoids and antioxidants by eating a variety of fruits and vegetables.
Supplements
Omega 3 fatty acids have been found to decrease inflammation. Increasing the ratio  of omega 3 to omega 6 fatty acids leads to lower levels of leukotrienes that are involved in allergic reactions. Eating fish high in omega 3 fatty acids like salmon and tuna or supplementing the diet with fish oil can be very helpful.
Quercetin is a bioflavonoid that has been found to stabilize the membranes of mast cells involved the allergic response. It is usually recommended in the prevention, not treatment of allergies so it should be taken during the entire allergy season.
Avoid or Reduce Exposure
Remove environmental triggers such as fresh flowers, dust mites or cats. Outdoor pollens are the worst in the morning and on dry windy days.
Air Filter
Consider using a HEPA filter which removes allergens from the air. They are inexpensive and can be placed in rooms you frequent such as the bedroom or office.
Stress Reduction
Putting extra stress on the body can inhibit the immune response contributing to a more severe reaction to allergens. Put your relaxation strategies into action.
Other therapies
Consider exploring Traditional Chinese Medicine, homeopathy or hypnosis for the treatment of seasonal allergies. There is some evidence that these therapies can be helpful. Let me know if I can help!
 
See Rakel, D. (2008). Integrative Medicine 2nd edition. Saunders.