A lot depends on how you react to the barking. What do you do?
If you react in a human, knee jerk way, by yelling NO you may be being considered by your dog to be approving of the barking and joining in. She's making a "go away" noise or an excited noise and in her opinion that may mean that mum agrees!
It will help to remove her from sight/earshot of the stimulus when you can. So, keep her away from the room overlooking the street when you can and especially when you go out. Leave a radio on to both mute outside noise (I was going to say "drown out" but that insinuates that you should play Led Zep at 100 decibels... I'm sure you get what I mean though!).
You can also teach her to bark... don't laugh, but teaching her to do so by holding a treat in front of her and making a bark noise yourself, rewarding her with the treat and praise when she copies you, can then enable you to teach her not to bark unless on command. I've taught my lab x to bark on command in just this way and know others who have done so, it does work.
Also, distract, distract, distract and praise effusively. Dogs, it is thought, react to reward... in her case her "reward" for barking is to see the postman/passers by go away. You know they were going to do so anyway, but she doesn't! In her mind she has barked and been rewarded by seeing the interlopers take notice of her and disappear.
If she's barking when you are trying to lie in, she may need her sleeping quarters moved, even if that means bringing her into the bedroom (in a crate if you wish, though introduce this gradually, never as a punishment and with the door open and toys/chews in there if she is unused to being crated).
A great source of advice for this may be your dog warden. He'd far rather you consulted him for help than came to you because a neighbour had complained. If you still struggle though, consult a behaviouralist (see the net for registered ones near you or better still get personal recommendations but check them out for membership of an appropriate organisation, the titles of which escape me atm!).