Most barbecue manufacturers recommend that your barbecue be cleaned thoroughly at least once a year. This is on top of your usual cleaning after every use.
Remove the grates and wash with hot soapy water.
Wipe down the connections, lines and lid (inside and out) with hot soapy water.
Remove old or dirty lava rocks and replace with new ones.
Check the burner. If it looks good visually, light the barbecue and make sure it is supplying an even flame. If not, then replace it. Most burners only last about 2 seasons, depending on how often you use your barbecue.
While the grates and lava rocks are out, clean up any grease or ash that may have accumulated on the bottom of the barbecue.
Check your fuel supply. To check the gas level in your propane tank, do the following: (1) Pour a cup of boiling water over the tank, (2) Feel the metal with your hand. Where the water has succeeded in warming the tank, it is empty; where it remains cool to the touch, there is still propane inside.
Clean the Venturi tubes that supply gas to the burner. Bugs, dirt, and grease can clog them over the winter and prevent full gas supply from coming through. Check your owner's manual for the recommended cleaning technique for your grill.
Check your cleaning brush, barbecue mitts and utensils. If any are worn or dirty, replace with new tools.
Reconnect and replace all parts and fire it up to see that its working well and is ready to grill.