If I was going to make a distinguishable flashback I'd have to look at the scenes I'm intending to flash to and from and make a judgement call. For example, extreme lighting changes often signify a massive time jump of some sort, but if both scenes were set outside in a field during the same time of the day that'd be kind of hard to do. I'd look at each aspect of the scenes major properties and decide then and there what to change, preferably at least two - off the top of my head I can think of lighting, colour schemes, pacing, action, location, cast, dialogue, etcetera.
Of course there's always the option of leading into it with dialogue or narration during the present-time scenes, such as the opening of Casino Royale where they're discussing the person who Bond killed and it's flashing back to shots of them fighting in a bathroom every now and again. In that I think they managed to distinguish effectively between the two using a massive difference in pacing and lighting - in the present-day scene it's very calm but tense, with them both sitting in chairs and discussing Bond's career (avoiding spoilers), whereas in the flash back scene it's a well-lit bathroom where the two characters are locked in a fist fight. There's also a massive difference in the sound, with one scene only having their voices and the other having an actiony soundtrack and the sound of them fighting. It's also worth noting the entirety of the beginning of this movie is in black and white until the opening animation, setting the entire section as a flashback for the rest of the film / a prologue.
Really it's all a matter of the scenes you're using in particular, and the style that you want to go for.