Suggest plans that are much easier for her to agree to than not (e.g. "coffee... next Tuesday." She's probably planning to cancel; don't worry about it.).

Cancel the night before, before she can cancel on you if she still remembers your plans, with a reason that piques her curiosity about you ("Doing an interview" / "At my friend's restaurant opening" / "My friend Amelia is going back to London"). Promise to make it up to her, but don’t suggest anything specific.

Use this if she flakes on you with a late text message, voicemail, or if she doesn't even contact you.

Pretend it never happened.

Pretend YOU were the one who couldn't make it.

Call or text to apologize.

It sounds absurd, but it works. Whatever you do, do not shake your inviolable frame that, despite any and all possible evidence to the contrary, you were the one who couldn't make it. If she insists on pointing out that she cancelled too (which is rare), act relieved.