I'm writing a first-person POV in which my MC has flashbacks/memories of previous lives. As these happen several times throughout the novel, I use italics for these scenes to help the reader understand it's not current time. At one point, in a previous life, she has a dream. Is there any rule, or tips for formatting this so people can understand it's not reality or present day?
First of all, there are no rules ;)
As for tips, there can be formatting methods to indicate this (say, use a deeper indent), but I would personally focus on textual/narrative methods instead.
Ideally, I believe, special formatting should not be used at all. I wouldn't even use italics. This is especially the case if these flashbacks can (and they should) be structurally separate. That is to say, if they are the natural beginnings of chapters, you don't need to use italics. If they can't be separate chapters, you could at least separate them with sectional separators (three dots for instance). With a bit of context, a reader can very quickly realize it's a flashback.
Trust your readers. Readers always "get it".
Especially if these flashbacks occur often, overdoing it with italics (or other special formatting) can be tiring.