I know this topic has probably been beaten to death, but I don't think I've seen this online yet:
I keep a cheap (like 2$ eBay brand) UV filter on the front of all my lenses while in my bag, and I don't use lens caps. That way I'm always ready for a shot, even if it means shooting with a mediocre filter. Better than nothing, quicker than a lens capâ€¦ When shooting something that I care about, I'll usually replace the cheap filter with a name brand filter. (mostly to please the professionals I know who INSIST on high dollar filters. I'm still undecided on thatâ€¦) I've also dropped my nifty fifty on concrete from a few feet and shattered a filter with no visible loss in IQ. When I have a second, I swap my "lens cap" filters for a CP/ND/whatever and keep the cheapie in my filter case. Problem solved.