Yellow, green, yellow-green or orange filter if there's going to be any sky in my shots (unless I want the sky to be white, which I sometimes do). Nothing less than B+W or Heliopan, even if I'm only shooting with a $20 lens.
Up until this summer, I used to have a UV filter on each lens at all times, but it decreased image quality somewhat, so now I'm simply more careful with the lens than before. Even after leaving the UV filter at home, I still didn't bother with lens caps though. My bag is fitted with the right sized compartment for the body and lens, and I padded this compartment with some cloth. The camera gets dropped in there and stays safe. There's usually a collapsed rubber hood on the lens, which keeps the front element clear of anything that could rub against it by a couple of mm.
By the way, a tip for dusting your front element: use disposable lens paper (Kodak makes it; probably other brands as well) rather than microfiber cloth. Since you never use the paper more than once, the chances of actually *adding* dust, dirt and oil to the front of your lens are practically nil. Furthermore, the paper is used not to wipe, but to lightly dab at the glass, so you don't have to worry about wiping marks either. And if the lens ever gets *really* dirty (i.e., children get their fingerprints all over the front element :bang: ), add a few drops of lens cleaning liquid before applying the paper. Works like a charm.
