Im going to post this but I want to make it clear I'm not advocating it for everyone, because grills may be different and what works for me may not work for you. This is what I do, as recommended by a grill salesman. Your mileage may vary, act accordingly.
I keep my portable in a vinyl bag that came with it. They no longer make my model and it's been a great grill for 8 years. When I'm done grilling for the weekend, I ususally put a piece of aluminum foil over the entire grate and let it run until it stops smoking, usually not more than 10 min. When you take the foil out, most of what remains is white ash, even down below. If you do this regularly you will avoid the greasy buildup and hence the attration of odor and vermin. Works for me, but take care trying it yourself. All the heat stays below the grilling surface and it can get very hot down there. Don't walk away while you are doing it.