A desert racing friend of mine and 20 year motorcycle mechanic actually gave me this cheap fix. Take an old inner tube of the same size and cut out the center strip about 3/4 inch, the area that would meet the rim band. Then take the remaining piece of inner tube and wrap it around the good tube evenly. Install both tubes together inside the tire. What this does is prevent the tire sidewall from pinching the good tube causing a slice. Rubber pinching rubber won't cut the good tube when you hit a rock or g-out, compressing the side wall. I have never had to finish a race with a flat since doing this and its cheaper and lighter than thick tubes or moose type tubes. Give it a try you wont be sorry.
If you are trying to tame your crf450 for woods you would better purchase a flywheel weight 17-19 oz. this makes the power roll on smoother, controlling traction and making the bike alot funner to ride. Definetly buy a good x-chain the stock one is as cheap as they get and will ruin your stock rear sprocket in a hurry, if it hasn't already. I like the tri metal rear sproket, light and long lasting, not much more than a good all aluminum one. Have fun