Until I came to this forum, and although I shot flinters for years, I never knapped my flints. When they got dull, I just replaced them. Now I get a lot more live out of my flints than before by knapping. The guys here told me an easy method.
With the flint in the c0ck (hammer) you just take a small blunt nail or punch and just catch the edge of the flint's edge and tap it with a small hammer or dowel. You just want to flake off the edge. And just move along the edge in small steps. Don't try to break off too much or you will break the flint.
I like to use english flints as they give me the best spark.
As for the frizzen, it takes a long time to wear one out. Just try not to get oil on it when you clean your rifle. If you do, degrease it with alcohol or something similar. Oil may cause it not to spark like it should and will do the same to a flint.