you don't want to use anything that will leave a residue on the dish as this will cause signal loss. there is an aftermarket heat tape that you can apply to the back of the dish that i have been told works well. you can put a plastic bag over the dish, but not the lnbf(eyes) of the dish. pull it tight to keep it smooth over the dish surface. the best is just to simply use a broom or a brush of some kind to just clear any build up. most aftermarket products can be purchased at hardware stores or dish retailers.
There is an aftermarket cover that works, but I lost the website that sold them. They even make then with sports team logos on them. Also cooking spray does work but you must clean and respray your dish every week or so. Best bet is a long extension cord and electric plumbing heat tape. You can attach the heat tape to the dish with an industrial adhesive (liquid nails, etc) and plug it in during inclement weather. Good luck and be careful up there.