How to add custom functionality and make upgrades easier

Hey gang,

Our developer has made a few tweaks to Membership and a few other plugins on our site. We are trying to keep all the plugins up to date, but anything custom requires making sure that the custom code isn't overwritten, then tested to make sure it still works.

Do any of the more experienced coders have a good system for pulling out some of the custom code into a separate plugin, or .php file, so that the main plugin can be updated automatically?

We had a developer in the past who did this for us for a few changes and I really liked the process. Unfortunately we parted ways to do different working styles.

Thanks!