Beyond your gun, a good set of goggles in working condition are essential to a game of paintball. Make sure that the lens is not cracked, the mask is tight and fitting properly on your face, and that you can move in it okay. It is very important to remember that paintball masks were designed to withstand the impact of a paintball travelling at 300fps. Although skiing or motorcycle masks and goggles may look safe to play in, they are not designed for it and will probably cause more harm than they will help. Your gun and goggles are not the only pieces of equipment you need to be concerned about.
Comfortable shoes can really make the difference towards the end of the day.
Your footwear should allow you to run full speed, provide excellent traction, and be very comfortable. Wear loose clothing - tight clothing can chaffe and be very uncomfortable after awhile. If you are playing on a cold or snowy day don't be afraid to pile on the layers, as you can always take layers off - There is nothing worse than paintballing cold and wet. Gloves are often overlooked unless it is cold out, but even in the warmer times of the year they can be a life saver. Personally speaking, getting shot in the bare hand really sucks, and can even lead to bleeding.