One of the best investments I made for finding mammals is a book on tracks, trails and signs. May set you back a few quid but a least you'll know what to expect to see. It'll save a lot of wasted hours.
Listening to birds will give you a good idea that something is in the area. To get an idea of what to listen for wait until a cat comes near your garden, the song birds will make very distinctive calls. They'll do the same for any other potential predator.
As a bit of an aside, a mate of mine who used to be in the Army said that they didn't wash for three days prior to any covert activity. I'm sure that I also read about a photographer who used to store his cam kit with dry leaves from the local woods. Don't know how effective that would be.