Do all of the following with the gas off, if applicable.
Clean the "flame tamers," right over the gas burners underneath the metal grilling grates. A skewer, toothpick or paper clip are good for making sure all the little holes in the burners are open and unclogged. There are also tiny wire brushes made for this purpose. Later, when you test the grill, peek to see if any holes are still clogged. Then, once the gas is off again, give those openings an extra go-over.
Empty the grill of all ash and debris from the previous year (remove the grate to do this).
Make sure that grease pan is empty! Ideally, you would have emptied it at the end of last season, but in case you forgot, this is a big one, as grease fires are a hazard. Check this about once a week if you grill regularly.