Hacksaw:

Go to your local MEC and get Tuffy tire liners. I have them on my mountain / commuter bike and after 3 years and over 7,000 km (4400 miles) of riding in all types of terrain...city and off-road, I have not had a flat tire once.

The liners are well worth the money. In fact after fenders and panniers, the liners are one of the best and worthwhile improvements you can make to your bike. If you have smaller tires/rims, the liners can be cut to fit.

See here about 1/2 way down the page