Get a good multi-coated UV filter--B&W, Hoya--and keep the lens cap for storage or transport. Like kuvvy said, I've never seen any degradation of an image due to the use of a good filter in 40+ years. If you shoot into the light source, you can always take off the filter for extra insurance against flare. Lens hoods also help protect the lens but a good filter will do an even better job.
I used to shoot news for a daily newspaper back in the days when newspapers were paper and actually printed news. My lenses got dropped, banged against things, sprayed with some disgusting substances and generally faced frequent abuse. I replaced a lot of scratched and broken UV filters during those years yet my beat up lenses had clean, unscathed front elements.