There are small things everyone should have when they buy a DSLR. A small reflector, perhaps a flash or a diffuser... and that list goes on and on. The list gets larger and larger the more time you spend in photography and some photographers buy gear until they are in debt up to their eyeballs.
I wanted to share a quick story about how I saved a thousand bucks with a small, often overlooked item. Every lens you own should have a filter on it, if it can support one. A filter is a small piece of glass or plastic that screws onto the front of a lens. Some are tinted some are clear, there are hundreds of filters out there. Some filters allow you to take longer exposures, some allow you to take a varied exposure. Every lens you buy should have a filter... even if it's a cheap UV filter, which you can find for under $10.
While shooting a wedding this week I had an 18-200mm lens that is worth about a thousand dollars strapped to my hip... I tend to zoom around at weddings, work you know? As I went around a corner I scraped the front of the lens across a rock wall :( snap! The rock chewed into the lens with a horrible grinding in my ears. The little UV filter saved my lens and took some serious damage from the wall. I screwed it off and continued to shoot the wedding without missing a beat.
A UV filter doesn't really significantly affect how a DSLR captures images, although it used to have more impact in the days of film. You can use the filter and it is still going to give you a clean image on the camera sensor. Some filters can produce lens flares that you may not like, so test a few of them out!
Camera stores will always try to upgrade you to a nice filter when you buy a lens. The main benefit I have seen those retailers spin is protection of the front lens element. And I swear by this as well now. Don't always buy the lens upgrade(s) they may offer you but a small filter over your lens can be worth a thousand bucks.