Personally, I like to use Gunk engine degreaser on sensitive areas, or oven cleaner if I need to be a bit more aggressive. Note that oven cleaner is an excellent degreaser, but it is corrosive (great for removing rust film) so you must be certain to wash all traces of it off with soapy water. For final prep before painting, I like to use a product called PrepAll (made by Klean Strip), but will also use mineral spirits if nothing else is available.