I had really horrible allergies, particularly to trees and grasses; the sinus drainage would caus me to get bronchitis several times a year. Then, around 1993, my doctor put me on fluticasone nasal spray (the brand name at the time was "Flonase") and the inhaler ("Flovent") with the same medication for my allergy-induced asthma. My situation has improved dramatically since; I seldom have bronchitis now. Both drugs were originally VERY expensive; fluticasone nasal spray is now available in generic form, and the price much lower now; Flovent has not yet gone generic, but the cost is well worth it to me. These require a script, but if your allergies are literally making you sick, or just plain miserable, it's worth checking with your doctor to see if they are appropriate for you.
There are also several good over-the-counter eyedrops for itchy eyes. For some people, a simple saline solution helps. For others, there are many brands of eye drops with a small amount of antihistimine in them that help a lot. Also, as strange as it sounds, be sure to keep your sinuses OPEN; when your sinuses are blocked, your tears can't drain. Your tear ducts, at the inner corners of your eyes, drain directly into the sinuses; if the sinuses are blocked, then your drains "back up" and your tears can't escape. Something like Sudafed (pseudoephedrine HCL) can help keep the sinuses from swelling. Most pharmacy chains sell their own version that's much cheaper than the name brand stuff, and works fine. In most states (maybe all??) you will have to be at least 18 to buy it, and present a photo ID.
Nasal spray can help for occasional sever blockages; get the kind that contains phenylephrine HCL. DON'T use it more often than it says on the bottle; your sinuses can develop a sort of tolerance and will actually become MORE swollen if you use it too often.
Next are antihistimine tablets, such as Benadry (which makes most people sleepy), Zyrtez, Claritin and other "non-drowsy" allergy preparations (which don't generally make people sleepy, but also don't work for many people). For those who ARE helped by the non-drowsy allergy meds, they're great, and you can get generics of those as well.
Some people swear by "neti pots," that use saline to flush the sinuses. If you use one of these it MUST be completely clean, and the saline solution MUST be sterile; there have been many recent of people using tap-water solutions who developed severe infections; a few died (the sinuses are VERY close to the brain) so always keep the pot clean (such as running it through a dishwasher) and use STERILE saline solution - NEVER use tap water.
A few other things that might not occur to you:
If you're going outside to mow the lawn, or work in the garden, or whatever, and you know your allergy sensitive, taking antihistimines or decongestants BEFORE you go out can help minimize your body's reaction to allergens. Once you're finished being outdoors, shower, and WASH YOUR HAIR. Pollen LOOOOOOVES hair, so don't give it a home. If you've been mowing or working in the garden, put on fresh clothes after showering (which you'd probably do anyway), and put the dirty ones in the washer IMMEDIATELY. Don't let pollen-laden clothing sit in your house - get rid of the pollen with a quick, cold-water wash.
Some people get a bit of relief by using indoor air purifiers; it's at least a refuge away from the allergens lurking outdoors. Also, don't let your indoor air get too moist or too dry; either direction cause problems. Between 40 to 60 percent relative humidity is a good range for most people. If you use humidifiers, dehumidifiers, or air purifiers, be SURE to change the filters regularly. The same is true for air conditioners and heaters (especially central air systems); filters can only do so much and if they're blocked, you getting no benefit, plus your unit is working harder and costing you money.
Beyond that, do your best to avoid things that "set you off," whether it's simply being outdoors, or staying indoors on high-pollen days if you can. There are dozens of services out there that will alert you to high-pollen days in your area via e-mail, text messages, and so on, so sign up for one of them; they're generally free.
For the record, I am in no way affiliated with any of the companies that make any of the products I've mentioned. I've battled allergies LITERALLY since I can remember, and I'm 58 now; these are things that I've learned the hard way; I hope they help someone else out there.