It's safe to try these two things together for a month: Flonase, as Jen suggests, and Claritin tablets (or the generic equivalent). You can also try a neti pot with saline solution as many times a day as you like.
If the problem is sinusitis or allergies, these things will improve the congestion.
However, among the causes for blocked nasal passages are permanently swollen turbinates--the inside part of your nose, way deep. This swelling can be temporarily reduced by Afrin nasal spray, but it's not safe to use it more than three consecutive days.
If the other stuff doesn't help and the Afrin does, then you need to see a doctor. There are out-patient procedures to reduce the size of the turbinates. I had it done about fifteen years ago and it was greatly improved for more than ten years. Now it's getting bad again and I'll get something done about it soon enough.