How to Improve Your Credit Score Quickly in the USA and Canada

I am planning to buy a house in the next year and I really need to get my credit score up. It is currently in the high 600s and I want to get it into the mid 700s to get a better mortgage rate. I am looking for some proven ways to boost my score quickly.

I have already started paying off my credit card balances and making sure I never miss a payment. I am wondering if there are other tricks, like being added as an authorized user on someone else s card or using a credit-building service.

Does closing old accounts hurt your score? I have a couple of cards I never use and I was thinking about getting rid of them to simplify my finances. I have heard that might reduce my average account age and hurt my score.

If you have successfully raised your credit score in a short period of time, I would love to know how you did it. Any advice for someone in the USA or Canada would be very helpful. It feels like such a mysterious system sometimes.