As always I use the least harshest approach possible.
I like to use car wash soap and an old mit or whatever brush fits in the area.
I have used Simple Green, a heavy duty degreaser and have even used Gunk Engine Cleaner on some really bad ones.
After cleaning the hinges, especially with a degreaser, make sure to re-lubricate them to avoid early wear. On my personal vehicles, I use WD-40 because it doesn't leave much of a mess.