About six months ago, my credit score abruptly jumped from the low 300s to the high 700s. The only two things that changed in my financial life were:
(1) I became listed as a joint trustee on my husband's revocable trust (and vice versa). The two trusts are of comparable size and each owns several sizeable accounts.
(2) I took out a second credit card and paid off a small (less than $100) balance, but only once.
Another puzzle is why my credit score was so low to begin with. I paid my credit card in full every month for decades -- it always had a four figure balance and sometimes a five figure balance, which were always well below the spending limit. (Mortgage paid off 20 years ago; no other loans of any kind, ever.) In contrast, my husband's credit rating was in the 800s. He had a second credit card for minor purchases, and was a co-holder on my card, which we used for major purchases. It seems to me that he got "credit" for what was principally my card, and I did not -- or is that paranoid?
This is just curiosity. It is unlikely that I will ever borrow money.