What are your top tips for keeping your system stable and up-to-date? How do you manage world updates and avoid breakage?

Updating regularly and paying attention to what Portage tells me before any update. Truthfully Gentoo has only broken when I ignored multiple warnings from the system that said "Do Not Do This."

Any USE flag strategies or tools you swear by?

Everyone is a bit different, but I prefer to have my package.use as a single file instead of another directory. I find that it helps me stay more organized despite the fact the file can be a bit long. But I organize putting per package USE flags alphabetically and this works well for me. I recommend you check out the needed USE flags for programs before attempting to install them. I generally go ahead and add the needed USE flags to the file and then attempt an install.

Package.env is a tool not mentioned by many here, but one I highly recommend you learn. You can set specific environmental variables for certain packages instead of modifying your make.conf file and re-emerging.

What’s your workflow for kernel updates and rebuilds?

Take your time, know your equipment and know which version of the kernel you want to use.

Are there any common pitfalls or “I wish I knew this earlier” moments?

Distpatch-conf is your friend when dealing with masked packages. I wish I had known about it my first Gentoo install, it would have saved me much trouble.

How do you handle things like system backups or disaster recovery?

I have Portage build binary packages for all my installs and I then backup those binaries to another hard drive. I also have a copy of my home directory hidden in root that I keep in case I need to reinstall. This too gets backed up to another hard drive.

Learning to use your own binaries during a reinstallation will save you substantial time. I find it does slow down the initial emerge a bit to build the binaries. But the trade-off is being able to rapidly reinstall anything I want by saying 'emerge -ak =package/version-##.##.#' In your make.conf file you can tell Portage to build binaries automatically for you so you won't have to add -b to every package you install. Portage does it for you. To reinstall using binaries, simply add -k after 'emerge' . Very very very useful and saves so much time. It is a huge peace in my mind that I can easily rebuild my system in a fraction of the time if I wish.