I always have a minimum of $150 ($100 + $50 bill) on me plus some $20s and $1s. I rarely use my credit card, but my wife uses here credit card even for small purchases.
When we worked, we saved 15 to 20% of our earnings. I retired in 1998, and my wife a few years later. We invested the money into Vanguard funds. Ten years ago, our balance was a little over $300k, and now our balance is double that.
I learned early in my working career that investing for the long term is the best policy. The next rule was "diversify." Many investment pundits recommend investing in bonds as one grows older, but I never followed that policy. Rather than picking individual companies, we invested in funds and let the experts pick the composite. Our choice was between Fidelity and Vanguard; I picked Vanguard. My wife's investment has more funds than mine; I have only two funds, she has about eight funds.