I pretty much never carry cash. If I really need it (like for a poker game) I'll stop at the ATM.

Probably the best way to do it is to budget all expenses, allocate some for cash and some for credit, add up all the cash expenses for a month, divide by 4.3, and take out that much cash every week. You could even keep a budget chart with you in your wallet so you know which expenses are for cash and how much.