My ENT prescribed a nasal spray called Drixine (google it) for me.
It is the most awesome decongestant I have ever used. It literally works in a minute or two, and keeps your sinuses open for hours at a time.
Unfortunately, you can't use it for longer than 5 days, but that should be plenty enough time to get over any illness you might be suffering from.
It is available over the counter here in South Africa, I'm not sure about other countries.