I have found that I do not need to change the blades as often if I clean my blade and oil it every so often. Doing this gets out all of the 'scum' and lint that accumulates under the blade. The lint under the blade will slow your blade down tremendously.
When my blade is not cutting like butter, rolling smoothly or I feel I have to press harder, I take the cutter apart and clean it. I take it apart and put the pieces down in the order that they came out, clean the plastic with oil and oil the blade. I flip the blade over, reinsert it and then I also put a drop of oil on the plastic piece that goes on top of the blade. I put it back together, being sure that the blade is secure, but not too tight. Note: Be sure you only use a drop of oil, as you don't need to go overboard or the extra oil will get on your fabric.
Doing this often, especially if you are doing a lot of cutting or cutting something with a lot of lint like flannel, not only keeps the blade sharper longer, but keeps your hand from getting tired, because you are not pressing so hard on the blade.