I'm finally in the position to have an actual knife fund. I take $30 from every paycheck (every 2 weeks) and put it directly into a savings account. It actually cost much more to have than interest it earns, so it's really just a different pot to separate it from money for bills and such.
Though it's not a whole lot, it adds up quick, if I save it. Plus, it gets the "round up" from each bank card purchase on my checking, so adds in $5-10 dollars additionally each week, or so.
If it's not there, I don't buy. Keeps me happy, and keeps the wife happy. So, if I want a $400 knife, I just have to save for a few months.
This fund is also used for other items I collect, or desire. New bottle of scotch, comes out of the account. New humidor or bundle of cigars, new silver round, new fishing pole, etc...