Write them down. Keep writing them down. Read up on dream interpretation, and you can actually interpret flashbacks the same way.
It doesn't matter if you watched Spider-Man the night you dreamed it or not. The form of Spider-Man can carry some significance to you, that it re-occurs in dreams and flashbacks. It can be interpreted like a dream whether it means the "trickster" archetype force working through your life, or a desire for a better perspective in your life and empowerment of like web-slinging through the city.
I mean, puh-leeze, a spirit guide I work with takes on the form of Captain Hook. Of course I don't take that as literally that I was Smee in a past life or other reality or something. Instead I think about how pirates have a connection with the perils of being outside the conventional way of life, and have a connection with the ocean (another important symbol for me through my whole life) and those are issues that I work through because they reflect and apply to my life right now. (Why can't my spirit guides be ninjas?? Oh, well.)
There are guided meditations that you can take to stabilize a conscious visit into the past life, but of course there's a trick to it, to be relaxed, and not get too conscious out it so you're not manipulating the experience too much.
Past life regression meditations can often ask you to focus on one detail or defining moment while embodied in the past life. Once you've started there, try to pry into the memories of the form that you embody, to get a sense of their childhood; then of how they died. Ask, what were the general themes of this life? What was the lesson that they're still stuck on and gives you access to their life because you have something to learn? Are there any people you recognize from that regression, that accompany you in this present life?
Another neat weird thing I have read about happening, then, is to dis-associate yourself, so you become yourself again, and the past life becomes somebody separate--and then you talk with each other. I didn't think that could be possible, but consider that it's just pure information that your brain is translating for you and then there's no need to worry about, "Hey, why aren't I counseling future lives then? How can this have been my past life and not still be me?" Those are logical questions being brought into a reeeally illogical level of reality, so I'd advise to just continue to interpret dreams, flashbacks, and meditations like as if they're all dreams.