Ignore me, I'm just brainstorming different ways to telegraph flashback sequences.
Different storytelling style. Like the obsessive moment-to-moment transitions of the origin sequence in The Dark Knight Returns. Definitely not a one-size-fits-all technique, and best used sparingly.
Different background color. You see this a lot in manga, where flashback pages will have a black page background and current pages will have a regular white page background. You don't see this much in Western comics because it used to be that full bleed used to be an extravagant expense, and then we flipped and nowadays every comic has all-black backgrounds. It's an effective signifier but also robs you of the aesthetic possibilities of altering the background color for other reasons.
Different coloring/toning style. You used to see this in old Marvel comics, where flashbacks would be in garish monocromatic schemes (usually pink or light blue). Occasionally you'll see this in manga, where flashback sequences might not be as heavily toned. On the extreme end, you'll get something like Supreme with the faux-distressed halftoned coloring in the flashback sequences. A big disadvantage is that this technique definitely calls attention to itself.
Different line quality. Stan Sakai uses this a lot in Usagi Yojimbo, where flashbacks are inked in a looser style with relatively few areas of solid black. Makes it nice and obvious that something is different without hammering it home, but you'd better be able to ink your work in two significantly different styles. Works best in black and white.
Different art style. Usually the result of two different artists working on the book, as in Supreme where you alternate between Rob Liefeld clones and Rick Veitch doing his best Wayne Boring impression. On the other hand, I've also seen books where the flashbacks had a painted/washed style and the current sequences were more traditionally done. Another technique that calls attention to itself, and one which may be detrimental to the overal aesthetic unity of the comic.
Different lettering style. You see this a lot in manga, where flashbacks will have a lighter or more scriptic font. This works best with fonts that aren't trying to imitate hand lettering, which is why you don't see it a lot in Western comics.
Different panel borders. An oldie but a goodie — replacing the panel borders with thought balloon shapes (or at least just the intro and outro panel edges). Classic, but maybe too retro for some, and there could be an issue differentiating between flashbacks and fantasies if you use this technique. Alternatively, you could eschew panel borders entirely.
Different margins. Paul Grist uses this in technique in Kane — the "six months ago" sequences have wider margins than the "current day" sequences. It's a bit subtle and may be missed by a casual reader, which isn't heped by the fact that the first use of the technique comes after a page turn. As the flashbacks start occurring mid-page their significance becomes more obvious. Then again, it's entirely possible Grist wanted those sequences to be confusing on first read.
First-person narrative captions. A staple of old-school comic storytelling, but perhaps a bit too old-school for modern tastes. For that matter, it may not be as distinctive as it used to be now that the narrative caption has replaced the thought balloon. Probably works best with combined with one of the other techniques. It might also help if the narrative captions are the only text/dialogue in the panels.
Artsy-fartsy techniques. Montages. Trippy collages. The whole sequence takes place inside a giant silhouette of the protagonist's head. Maybe combined with one of the above techniques. Just make sure you have the artistic chops to pull off anything that's ambitious.
Don't do anything, just let the audience puzzle it out later. Also known as the Pulp Fiction approach. A good way to confuse the hell out of everyone and make a lot of people angry. The story you're telling had better be damn compelling if you're going to pull this one off.
Stop pretending you're an artiste and just tell the damn story chronologically. Where's the fun in that?