Try to NOT blow your nose; this is going to sound a little gross, but it was an ear, nose, & throat doctor who told it to me, when I had a string of sinus infections. Snuff the mucus back (inhale, hard, through your nose) and then cough or spit it out. One of the things that happens when you blow your nose is that the mucus backs up into your sinuses and ear tubes (eustachian tubes), and it sits there and bacteria go to town on it and, there you go, an infection. You want to get the mucus out of your head, not have it camp in your sinuses.
My doctor told me to use a saline nasal spray daily to flush out the sinuses. I try to take vitamin c and wash my hands often as well. Not sure there is any way to guarantee that you won't get a sinus infection though.