My daughter was on antibiotics every 6-8 weeks for a chronic sinus infection between age 11 and 23 months... Allergy medication didn't do anything. Oh and all the running around for x-rays on her sinus cavity... reoccurring eye infections... ear infections... I'm sure you know this drill... Thing that turned the tide was a nasal sinus rinse. It was prescribed by her ENT. At first I had to dissolve a antibiotic creme in salt water. But now, we just do it with salt water (boiled for sterility, then cooled).
Perhaps you can familiarize yourself with Neilmed's pediatric sinus rinse kits: http://www.neilmed.com/usa/products.php
It comes with a squirt bottle for the solution and another product of salt packets. (I will add that doing a nasal rinse on a small child is controversial since there's a risk of water in the lungs. As I said, we were prescribed this method under doctor's care... Something to mention at your ENT visit?)
The sinus rinses have made a **world** of difference. Huge improvement! She had to go on 1 antibiotic this past summer for congestion that developed into brochiolitis, but besides that we've managed to keep the oral antibiotics away since March-08... Of course, I have to give candy rewards for her cooperation with "booboo's in the nose"... (She hates it! Makes you feel like you're doing some waterboard torture on your child! ugh, it sucks...) And its a 2 parent endeavor (one to help position the head, the other to squirt). We do it every time I notice congestion starting to buil/runny nose (when its clear, I don't wait or it to turn colors).
We had a recent development of an ear infection from her congestion, but with daily sinus rinses, non-oral ear drop antibiotics, and eye drops, we were able to avoid oral antibiotics...
Let me know if you need more info. (I feel your pain!)