Most of the time the congestion is caused by a viral syndrome. Allergies and teething can also be culprits making you congested.
For older kids don't minimize the value of chicken soup!
For night coughs try a bit of Vick's Vapo-rub on the feet!
The second I feel that little tickle in my throat or any tell tale signs of an illness I take a Zinc cold therapy lozenge (Walgreen's generic taste better and are cheaper) and a Sambuccol lozenge twice a day.
Another very good thing to do for folks who are old enough is to use a Neti Pot or sinus irrigation
Good Hand washing is of course essential.
Wishing you a back to school season that is kind and gentle.