Those of us who love cats, but are allergic to them, constantly strive to find new ways to reduce our allergy symptoms. These widely-diverse products each help in their own specific applications, and the costs vary widely, from the HEPA air purifiers, to antihistamines. If you are like me, you'll probably need to take a "shotgun approach" toward reducing your allergic reactions to cats.
Some of these products are equally effective for helping asthmatic cats.

image courtesy of PriceGrabberCleaning the home of dander is the first priority in reducing allergens, and the best place to start is the air itself. If you have used other, noisier air purifiers, the Blue Air HEPA system will be like a breath of fresh air. The
Allergy Buyers Club gives Blue Air a "very good plus" rating.
The largest unit (Blue Air 601) cleans the air in rooms as large as 679 square feet; the smallest (Blue Air 402) can handle rooms of 365 square feet.
The downside is that these units are large and take up space, so weigh that factor against the advantages of a superior product that is relatively silent in operation.

Courtesy of Price GrabberRegular vacuuming of carpets, drapes, and furniture is highly necessary for keeping airborne cat hair and accompanying dander under control. The Sebo was rated very highly by the Allergy Buyers Club in the mid-sized cannister range of HEPA vacuums. A 3-step filtration system helps ensure that dander and airborne particles do not escape the vacuum. Rubber-coated wheels rotate 360° for easy maneuverability, and an ergonomically-designed handle helps to prevent hand and arm strain.

image courtesy of PriceGrabberA steam cleaner, in my opinion, is essential for both hard surfaces and carpet which act as magnets for cat dander. The Bissell Quicksteamer is a lightweight bagless cleaner with an easily removable brush for cleaning. The eight-inch path makes it easy to get into nooks and small spaces for thorough steam cleaning.

Image courtesy of PriceGrabberExperts recommend frequent (at least weekly) bathing of cats if you are allergic to cats, to reduce dander. For humans and cats who can't deal with bathing this frequently, Allerpet is an excellent solution. Before applying, use a fine-toothed comb to rid the cat of loose hair. Then moisten a wash rag with Allerpet and apply it first against the lay of the hair, then with it. Use once weekly for best results.

Image Courtesy of PriceGrabberOkay, you've taken steps to reduce your symptoms of allergies to your own cats, but what happens when you're visiting a friend with cats, or doing volunteer work at a shelter? Most people I know say they dose up on antihistamines before such a visit. Unlike antihistamines, BioAllers is a homeopathic remedy, and is useful for both short term relief and long term desensitivation to the allergens in cat and dog dander.
Benadryl has been my choice of antihistamines for years, for severe allergic symptoms. I truly never leave home without it. Be sure to check with your doctor before taking any form of OTC medications, and use particular caution before driving or using machines when using antihistamines.
Allergen Injections
This product cannot be purchased OTC, but can be formulated by an allergy specialist, using cat allergen extract from your own cats. Some allergy sufferers have received long-lasting benefits from allergy injections. They may not help against strange cats, but they can go a long way in making you more comfortable at home. Check with your own allergy specialist to see if injections might be feasible for your allergies to cats. (Tell him/her that getting rid of the cat not the first option.)