Use a cloth game bag, it will do a great job of keeping dirt, insects, and truck bed crud off your meat. Never use plastic, it draws heat and can cause rapid spoilage.
Spoilage mostly happens because of heat. To combat this problem skin the animal in the field (the price of the hide is peanuts compared to the meat of the animal). Submerge it in cold water (a creek, river, or lake) If cold water is not available try to remember to buy 3-5 blocks of ice. Put as much ice as you can into the cavity of the animal.
Carry 2 sharp knives in the field. Use the first to remove legs and innards, and to cut the cartilage in the brisket to the pelvis area. Also be sure to get the windpipe out. Use the 2nd knife to remove the hide. 3 inch or 4 inch knives are are plenty long enough for these chores. John also suggests carrying a small saw, because there are three places that should be split, so air can circulate and cool the meat. The brisket at the bottom of the chest, the pelvic bone between the lower haunches, and separate the ribcage from the front shoulder (not completely just enough to get the heat out).
Another way to keep flies and insects off is to sprinkle 2 ½ to 3-course pepper on your game spray Pam on the hide and meat.
Over the years Peterson has seen successful hunts turn bad because the hunting party didn’t take the necessary steps in preparation after the animal was down. Take his advice and chances are it won’t happen to you.