When glass looks weather-beaten and etched, you may want to consider replacing it to improve visibility and driver safety. Before you decide to replace any piece of glass, you should clean it, inside and outside, using a quality window cleaner with either newspaper or a chamois ("shamy") to remove any grime and greasy dirt from the glass. A mild abrasive cleaner, like Bon Ami diluted with water may be needed to remove tarry smokers' residues.
You may be able to remove minor scratches in the glass by using toothpaste (its fine enough for polishing your teeth, right?) on a clean rag. If there are many minor scratches, typically from driving regularly through blowing sand (or dusty construction sites) you might remove them with a glass-polishing kit. The kit contains a polishing powder that mixes with water, and includes a felt buffing wheel attachment for your portable drill, which you use to apply and work in the compound.
Small chips can be repaired using an inexpensive kit. A suction-cup holds a special jig over the chipped spot, which holds a pressure screw and a mixture injector device. You repair the resin and its hardener (like an epoxy) and pour it into the injector. Pressing on the injector's plunger, forces the mixture into the chip to repair the glass.