More then likley it is allergies. You can see doc and get tested, you could start taking an OTC allergy med. Start tracking your diet to see if a food or food group trigger thing(for myself it was dairy).
I'm sorry, but you can clean/rinse your sinuses for some short term relief but if it is allergies, it's meds.
You can go the more more shower, air purification, the hypoallergenic bedding and such, and all of that will help. But the only way to get around meds is to get tested, then hope it is something you can avoid. More often then not, it isn't.