One of the things people tend to forget is that a flashback is just a scene. As in, it's just a regular scene; the only thing that sets it apart from the rest of the story is that it's out of order chronologically. Still, it really shouldn't be any different from the rest of the story. I mean, I guess you could italicize it if you really want, but the entire breaking out of perspective thing? It's abrupt and feels like the flashback isn't part of the story, if that makes sense.
Now, on to the question for how to write it. Personally, I don't like the flashback tags because it feels like it beats the reader over the head with something that should be obvious through the narration itself. That and it tends to break the flow of a story because they frankly tend to be out of nowhere. What I mean to say is that rather than using tags, try using transitions. Have the narration say something along the lines of, "John Doe closed his eyes, and as he drifted off to sleep, he began to recall the events of a few months ago." Then, break the scene and move into the flashback. Alternatively, if you're doing it in first person, you could start off the flashback by having the narrating character say, "Five months ago, we were in a similar situation" or "I remember when" and continue on with the rest of that particular memory. Of course, there's other ways you can transition, but I'm just offering pretty basic ideas.
(Incidentally, concerning the perspective question, it depends on what you want to do. I personally prefer third person because I find it easier in some ways to remove myself from the characters and allow myself to show the reader something a single character would never have a chance to see, but other people might want first person in order to get in the minds of a single person. In any case, once you choose a perspective, you really should stick with it. There's only one work I've read that switched perspectives, and it did it to show a division between four different factions in the story. In other words, it had a really, really good reason for doing it like that.)
As for whether or not characters should acknowledge flashbacks, that depends on how they're happening and what the tone of the story is. If it's a humorous tone and the character is awake but merely spacing out, it's entirely possible for the flashback to end with another character snapping them out of it, only to have them say something along the lines of, "Sorry. Spaced out." (This happens a lot on Scrubs, for example.) If it's a serious fic and the character actually stopped to recall whatever was in a flashback, you could either have them acknowledge it (if the revelation in the memory is particularly important) or just have it be shrugged off (if the character felt like it wasn't important or if they didn't focus on it -- probably due to remembering it in their sleep or something similar). However, there's also the possibility that you could simply tell the story out of order -- that is, bring in a scene from the past and plop it into the storyline where it's relevant (like how half the film Memento is presented), so the characters wouldn't acknowledge it because they're not actually stopping to remember anything. It's just that the narrator (who might be a separate entity) is recalling things by folding the story back in on itself.
TL;DR bit short, the last few questions (concerning what perspective you use and how you do it) is all up to what you need for the story. Just remember that, no matter what perspective you do it in, you've got to do it smoothly -- as in, good transitions so that the flashback fits into the narration, rather than sit out in the middle of nowhere, surrounded by tags.
Hope that helps a little, and sorry for the wandering thoughts every so often.
