So, in watching Firefly the other day, one of the episodes involved a lot of flashback, and flashback to varying depths. The narrative starts out towards the end of the story, and uses flashback to explain the immediate past; but it also flashes back much farther, to the "distant" past. More nostalgic stuff, like how the crew met, that sort of thing. And, since I have a story (my week six clarion story) which played with narrative timeline a little, I decided to pay a little closer attention to how they were achieving the flashback without confusing me.
Some of the tricks were visual, which I can't mimic directly on paper: the deep flashbacks were shot in a filter, so everything had this glorious golden/green tinge and was very soft-focus. The current timeline featured a character alone and wounded on an abandoned ship, and was full of blue and black colours. Easy to pick apart the immediate past from the current timeline by the presence or absence of the other characters.
Another trick, though, was using objects to link the closing of the current timeline with the opening of the flashback scene. A character opens a door in one timeline, then suddenly we switch POV to find all the characters rushing through the opened door in the other timeline.
I remember Scott talking about this kind of trick at little, at Clarion. He was suggesting I write my week six story in reverse chronological order, and recommended a script (Betrayal, I think it was) which did precisely that. Of course, being the obviously-prompt student I am, I have yet to look this up. (Yeah, I know, it's next on the list.) I may be misremembering, but I think he mentioned there was always something, a tiny motif or object, which linked the closing scene to the upcoming scene.
Which leaves me curious now as to why this helps ground the reader. It's a thread, I suppose, and gives the meatpuppet mind something familiar to hold on to. (Silly meatpuppet mind.) Maybe the trick is picking an object that's central to the emotional tension of the moment?