Unless you use them adeptly, flashbacks are a great way to bore or irritate the reader. The best example by far is Naruto.
If you think there is a solid case for using flashbacks, it's best to adhere to a consistent structure - e.g. alternating paragraphs jump back to a past time. A lot of people use italics but I feel this is a clunky convention, plus you can't typeset in an exam anyway. You can use tense to discriminate between past and present: "I head to the corner store" vs "I headed to the corner store". Present tense doesn't always have to correspond with the present either, you can apply it to the flashbacks. But it provides an easy way to determine between the two.
Or make it clear via a big discrepancy in setting, presence of different characters, et c.
My personal favourite is to use dialogue and direct speech to present past experiences. But as I said, be careful with flashbacks. A lot of the time, like using dialogue instead, you can use something else and get a greater effect. Hope that helps.