Take the unit outside, and set it over a tarp. Spray barbecue cleaner onto the greasy areas in the cabinet, and allow it to soak in. Then put on safety glasses and rubber gloves, and scrub the cabinet with a wire brush. Wipe away the residue, and repeat the procedure until the cabinet is clean. If the metal grill inside has become rusty, you can replace it with a new porcelain grill.
Porcelain is easier to clean and less likely to rust.
Be sure to get the correct size for your barbecue. If your barbecue isn't getting as hot as it used to, you probably need new lava rocks, which help radiate the heat evenly. The rocks last about 12 to 18 months and should be replaced every year. If the side burner cover or the bottom of the barbecue has gotten rusty, you can easily clean and renew it. First, scrape off any rust flakes, and sand the metal smooth with 100-grit sandpaper.