Personally I don't like the 'ask inside' version because I probably wouldn't go in. However if it was a good/interesting offer then that would make me go in. If that makes sense!
Defo put the offer on the poster Pete. Generally speaking, people are lazy and won't go in unless they're prompted to by a good offer. If it just says 'come in and ask for details' customers won't bother when they've just passed another salon with a 50% off poster in the window! Well that's my humble opinion anyways!
Definately plaster the offer all over the poster! Most people wont go in and ask. I think you should grab their attention straight away and give them a reason to walk into your salon!
I agree - make the offer clear in the window - I wouldn't go in and ask about special offers.
I wouldn't worry about competitors seeing the offer - if they copy it, you should be flattered. If they better the offer in their salon it means they're worried about the competition.
On a different note, does the salon your wife owns have sunbeds? I'm after some tanning accelerator cream stuff?