I do it like Devil; make it clear right from the first paragraph whose viewpoint the chapter is told through. Each character should have a distinct voice, even in third person.
Perhaps it would help you (it helps me) to write each character's story seperately, then weave them together on the next draft? I do that, but in reverse order: I write the first draft straight through, dropping in POVs when it seems right, then on revision I look at each POV character's story individually, looking for large gaps and personality changes (!!! gah.), and I'll end up adding scenes and deleting scenes, and then I have to stich the whole thing back together again.
You could also develop a pattern that the reader will get used to, to clue them to who's on stage next. Like give the main MC two chapters, followed by one of lover and one of pal, followed by two more MC chapters, one lover, one pal...but you still have to establish early who is doing the talking, and that's best done by distinctive voice. Or pronouns, or other characters speaking to the POV character by name, or bringing up something that happened only to the POV character, or using the feelings of the new POV character about the last POV character to make it clear there's been a transition (deosn't work real well with schizophrenic characters, mind you), or having the characters in different locations so you can use the location as the chapter heading instead of names, because you've already established that Sam is the character in London, and George is the character in Iraq...I'll stop now. You're a creative person. Get creative with it!