There are a wide range of safe cleaners to use on exhaust tips such as degreasers, all purpose cleaners, dedicated wheel cleaners and different metal polishes. Experiment around and find out what products work best for you. I personally like to use the degreaser by Chemical Guys El Diablo and P21S Metal Polishing Soap but again a lot of different products work.
After the initially cleaning you will need to polish the metal. I’ve tested several products and Chemical Guys Metal Shine is so far my favorite. Ever since using it cleaning my exhaust tips has been quick and easy. After polishing you can use a metal wax to help protect the tips. I’ve never had a need for this type of a product so I can’t speak to its effectiveness.