Nasal irrigation, +1000. You don't need to spend money buying the over the counter saline sprays, or neti pots for that matter. Buy a plastic squeeze bottle at Wally World or Walgreens. Buy some kosher salt, or table salt that does not contain iodine (iodine in salt burns!), and some baking soda (the baking soda acts as a buffer). Use maybe 1/4 teaspoon each of salt and baking soda per 1/2 cup of water, experiment and find out how much works for you. Use warm water. Fill the bottle, shake it well so the salt and soda dissolves, tip your head back and fill up both nostrils. Try to hold it as long as you can, you might try gently exhaling through your nose while the solution is in, to make it bubble and percolate. Tip your head forward and let it drain, then repeat the process a few times, or as much as you can stand it. I've heard that neti pots are good for irrigating while facing down, letting the water drain from your mouth. The process may bother you at first, but you'll come to enjoy it, and wonder how you ever got along without it. Do it several times a day when suffering bad sinus symptoms, and once or twice a day as a general thing. Keep the bottle clean, of course. Some people use only distilled water.
A variation I've heard of is hypersaline nasal irrigation, where you put a LOT of salt in the solution. You should not swallow this solution, just hold it in as long as you can, then let it drain from your mouth or nostrils. The salt supposedly helps draw out the gunk. Blowing your nose when your sinuses are acting up is not a good idea, the pressure actually drives the infection deeper. Try inhaling sharply instead to pull it out. Finally, if you have sinus problems, you shouldn't take antihistamines. They tend to dry out the sinuses, which leads to infection. Keeping the sinuses moist helps prevent infection. Instead of antihistamines, take medications with decongestants only. Guaifenesin is a great pure decongestant. Be sure to drink plenty of water with it.
Oh, try sleeping with a humidifier or vaporizer running in your bedroom, it helps as well.