In a class I took last year, I learned a tip about making the blades less likely to develop nicks. After making a cut, I (and many other folks too) use the rotary cutter with blade exposed to push or slide the just cut piece away from the ruler. By sliding the blade along the surface of the cutting mat, this apparently causes the blade to nick. The class instructor advised that we never use the cutter to push a piece of fabric. Hope this makes sense. As far as changing blades, I usually wait until it's just not cutting smoothly or I end up with a spot or two where the blad did not cut thru both (or all) layers of the fabric.