Once you have a diversified wallet of credit cards, you need to know which one to use based on what you are charging or on what rewards you want to accumulate. Some credit cards offer extra points at restaurants, gas stations, supermarkets, etc. while others are linked to specific loyalty programs in which you may want to accumulate miles/points.
Let your spouse/partner in on the plan as well. As Rob can attest, I get really annoyed if I see on a credit card bill that he used his AMEX at a restaurant instead of his Chase Sapphire Preferred. (We earn 2x the amount of points at a restaurant using our Sapphire instead of our AMEX.) It is funny to be next to him at a store, restaurant, hotel, etc. as he hesitates to take a card out of his wallet to pay a bill. As the card comes out of his wallet, I see the sweat on his forehead as he tentatively looks to me for approval. 8 out of 10 times he pulls the right one. We are getting there.