I found a transparent bra for my 90s 300 ZX. It was a fine piece of plastic that protected my Z and it was hard to tell it was there. The company who made it however has gone out of business and I haven't seen another one in years.
There are a couple of companies who make this stuff that sticks to your paint. One of them makes StoneGuard (a plastic film made by 3M) which is a film that you install on your paint, claims are that it won't hurt your paint when you peel it off but I'm not so sure that is true. There is another company (I forget it's name) that claims to have the same product for a lot less money. I believe Dali Racing has the stuff in stock (at a dear price).
I have stongard on my m3 and ive found it to be the best. its a clear plastic film thats cut specifically for the car and sticks on there. its pretty much invisible.. i like it a lot. only con is that after a while of heavy washes, uv etc it yellows a little bit