I usually sever ties and throw away all their crap.
What this guy said (/wave Lukas, haven't seen you in a while).
I never kept anything from ex-girlfriends, simply because I didn't want it to be a talking point with anyone new. Yes, Stine and I are aware of our ex's, and of our past histories with those persons, but there was never a reason to make it a publicly known thing.
Likewise, if you're the one being broken up with (in my case invariably), it kinda makes it easier to let them reside in the past -- I had three ex-girlfriends prior to meeting Stine, and all three of them broke up with me for increasingly dumb, trivial reasons. I had no desire to see/talk to any of them following the break-up, because I simply didn't care anymore.
The other way around though, is kinda different. As the person doing the breaking-up/off, it opens you up for problems unless you take steps proactively to quell them before they rise.
Namely, the one I saw mostly from my friends was when a girl breaks up with a boy and says something dumb like "I'll always love you" -- it's about the dumbest shit thing to say, because it gives false hope that there is still a chance, thereby opening this scenario when said girl begins dating a new boy, and meets Mr. Ex-Boyfriend on the street:
<Mr. Ex> Hey you!
<Girl> Oh, hi! How have you been?
<Mr. Ex> Not bad, not bad. Who is this?
<Mr. NewGuy> I'm her boyfriend, Mr. NewGuy.
<Mr. Ex to Girl> But but, you said you'd always love me...
<Girl> ...
See? Giving any pretense of false hope in an effort to "lessen the sting" ends up being meaner than just saying "I don't care that way for you anymore" and letting it go at that.
However, as for any stories of psycho ex-girlfriends, I don't really have any. One of my ex-girlfriends ended up dating my best friend
long after she'd broken up with me, and he was just the type of guy she needed too. They've since broken up after a two-year relationship, and I'd hoped it would have lasted because she had issues, and he had the means to resolve them.
No, most of my psycho stories are from when I was actually dating these girls.