I try to keep the guys exactly the same personalities in every story I write. Therefore, they have the same backstory. So you'll see me reference a backstory multiple times - like Brian and Nick hearing the single the first time in the homegoods dept of KMart, things like that. I tend to use the same "extra" names, too, like their bus drivers usually are named Chris, for example. I also repeat security guard names - some of which I know are actual guards and others I've made up (it's kind of sad, but I don't always remember which ones I've made up and which ones are names that actually exist). I like using the last names of other celebs for other extras - for example for lawyers, doctors, etc., unless they're a real person, their names are probably somehow related to some celeb or character I like. It just helps me to remember who's who.
I have crossed a few stories before by using the same character sketches. For example, I used the character sketch I created for The Goose from Saving the Day for Blondes in the bullies in the jail in Fix, but he went by a different nickname that was in the character sketch. I've also used Pop Stuff Online in more than one story, and I've used Amanda Golde as paparazzi more than just in the Something Beautiful stories - again, though, usually without actually using her name.
LOL Unfortunately for my readers, I see my characters like actors so I know when they're being used more than once but the reader may not because they don't know that I'm using the same character sketch.