GMC C1500 ENGINE POOR MILEAGE

I Have An 89 Sierra 1500-350 Engine- What Can I Do To Get The Best Possible Fuel Mileage Out Of My Truck-

I Have A 1998 Gmc 1500 4x4 With 119000 Miles. Truck Runs Fine, Starts The First Time But Gas Is Being Consumed Quicker Over The Last Year...