As the tires wear, you will notice a flatspot developing on teh center of the tire. When the flatspot gets fairly wide, it's time to replace. You'll get to know with experience what a large/small flatspot is. Some refer to the flat spot as 'tabletopping'.

When the tire is fairly warn, you'll notice that the tire flats by the smallest of debris. Carry a patch kit with you. Heck, you acn repair 6-10 flats with a kit. Would make it tough getting stranded in the mid of nowhere. I carry 2 tubes but always close to help on the trail I frequent.

Another tip is to carry a dollar bill with you. Use a $1, not a $10...I've saved myself from walking a few times when a tire has split. Use it as a boot to cover a large hole. A large hole will suck the tube right out resulting in another flat rather quickly.

Remember, a new tire can split as quickly as an old tire depending on the debris. Always carry the dolluh!

Good tires, expensive tires, new tires, cheap tires, they will all flat, be prepared.