Memorize important phone numbers (eg, close friends and family). Memorize at least one of your credit card numbers.
It's not uncommon to find yourself in a situation without a phone or wallet. Can't tell you how many times a would-be tricky situation was easily resolved by having a particular number in memory.
Those pieces of information have saved me a lot of trouble on several occasions!
I always dial my husband's mobile number by hand, and usually my close friends and family's numbers, too.
Credit card: I try entering the number, expiration and check digits from memory, then check it against the card before submitting the order. When the card number changes (bank decision or mine), I learn the new one by learning the differences - the first four digits are almost always the same, as that's a bank identifier. With AmEx, the variance was only in the last five digits, and was slight.
It's not uncommon to find yourself in a situation without a phone or wallet. Can't tell you how many times a would-be tricky situation was easily resolved by having a particular number in memory.