I know this story is a bit old but I just have to comment. I get frequent sinus infections and have not taken antibiotics in years. Further more I've only taken 1 sick day in the past 2 years. I'm not a doctor but this is how I think this all works. Antibiotics are delivered by the blood, under normal conditions your blood does not empty in to your sinuses. This is why at the START of an infection they do nothing. But once an infection gets bad; like I have a fever, headache, and I might pass out bad, that is when the infection has broken down the wall of your sinus and is getting in to your blood. At that point they can quickly work as you have noted. You may have noticed that a day or two before your feel better you see some blood in some very very colorful mucous. The green/yellow/white mucus is the infection, the blood is where it connected to the wall of your sinuses.
So what I do is as soon as I feel my nose get stuffy(not blocked just a little hard to breathe) and my mind not working at 100% (this is NOT easy to notice. It's taken me years to get close to right on this) I start to take long (Typically an hour) hot (as hot as I can with out it hurting) showers and use sinus rinses to try and push out the gunk. This is all to loosen the infection so i can get it out before it grows big enough to get me really sick.
FYI: Be careful when doing the rinses, if the water is not boiled or distilled you could get something worse from it. I also stay clear of decongestants, they seem to dry my nose out. This will only keep things there longer since dry mucous is sticky mucous.