One of the most effective and cheapest options to increase off-road traction is to drop the pressure in your tyres.  Yes, that's right a small decrease in air pressure can make a significant difference.

Lowering tyre pressures will give you more traction off road, whether you're in sand, snow, dirt, rocks, or mud. Depending on you tyres/wheels and weight of vehicle, you can go as low as 12-15 PSI [of course with bead locks, you can go much lower].   Even airing down to 20 PSI will make quite a difference!  When running pressures lower than 20 PSI, keep an eye on the tyres performance; popping a bead can be a pain especially when it happens in an awkward spot on the track.

Remember that the most important thing about lowering your pressures for off road use is to increase them prior to driving at speed on sealed roads.   If you drive at low pressures on-road, the tyre will produce more heat and handling can be dramatically affected.