Well, as somebody who's had more sinus problems than Crohn's problems, I have lots of experience with this. Unfortunately.
Anyway, the steroid nasal sprays can help a lot but they may take time you don't really have. Still, you might ask your doctor for these, especially if you have allergies and/or are getting lots of sinus infections.
Most of the antibiotics will give you D. Probiotics along with them may help that.
What I would really recommend is taking Sudafed PE, or, preferably, the older, original Sudafed. Get the generic at a pharmacy. Use it regularly for a few days.
At the same time, be sure to drink lots of warm liquids - warm tea, or even just warm water.
Breathing in steam through your nose helps. Boil a pot of water and hold your head over it and breathe in the steam through your nose.
And lastly, either get several bottles of saline spray and use it at least TEN times a day, if not more, or else get an irrigator, like a Neti Pot, and use that. If you just get the saline spray bottles, use it like crazy. You can't overdo it, so go nuts. 10 squirts a time, 10 times a day, or something along those lines.
Also consider Mucinex, which thins the mucus. Basic Robitussin is the same ingredient and does the same thing, just it's in liquid form instead of a tablet. So either will do.
The point of all of these is that a sinus infection occurs because the sinus cavity is plugged up and not draining. All of these things will help decongest the nasal passages and thin the mucus so the infected sinus will drain. Once it starts draining properly, it will often fix itself.