What is the best way to display magazine ads in my portfolio?

Unless you designed the whole magazine, I’d just show the ad (if the printing quality is good) and tuck the cover behind, showing just the magazine title and date if possible.

Sometimes it’s easier to get a look consistent with the rest of your portfolio if you take the files (I’m assuming the work is digital I guess, which may not be true) and get your own copy printed.

This is especially true for full page ads or something that runs close to the spine that may be tricky to get cut out cleanly.

As a bonus you get control over stock, size, etc…