As has been mentioned, you can buy special 'one-way' films to put on windows. As has also been pointed out, they're quite expensive and it's worth paying a specialist firm to fit them, because it's quite difficult to get it exactly right.
If you're determined to apply it yourself, here's a suitable supplier:
http://www.abodewindowfilms.co.uk/acatalog/Win dow_film_Privacy_Mirror.html
However, nobody has yet referred to the laws of physics which make these films (and all 'one way mirrors') work. They rely on one side of the glass being far more brightly lit than the other. So, for example, they can't work on windows which divide internal offices, because the lighting levels will be roughly the same. If they're applied to external windows they'll achieve their probable desired effect during the day (i.e. people inside the building will be able to look out but those outside the building will only see mirrored windows). However, at night there will probably be electric lighting inside the building, while it's dark outside. This reverses the effect, so that those outside can look in but those inside can only see a mirrored surface.
When I ran a railway station, some office-bound wally decide to spend a large amount of the company's money having this type of film professionally fitted to the windows of the station supervisor's office. This meant that my colleagues and I were given an element of privacy during the day (which was a waste of time because the office was open to the public anyway). At night it turned our office into a 'goldfish bowl' and meant that I'd not got a clue as to whether the train on the adjoining platform had been correctly despatched, because I couldn't see out!
Chris