It's not as hard as some here have made it out to be. I think it is simply a matter of having the guts to say what is on your mind when the topic comes up...and also to bear in mind that it's really nothing to be ashamed of (as long as it's not taken to an extreme...some things I have seen here would obviously shock a gal to bring up, so use judgment).

I do remember what I first said to my ex-wife, and have said it a few times to other gals since. It's not hard to make an opportunity to bring up the subject of her hair...sometimes it is simply complimenting it, or even touching it because it's begging to be touched (or even making an excuse like you were pulling a blade of grass out of it or something). My ex had a little longer than medium-length wavy red hair when I first met her, and I had complimented it and touched it at one point, and said simply "I have always adored girls with long hair." That was it. She was kind enough to allow it to grow for me, and I was happy to help take care of it.

I believe gals love it when you are honest, and if you are honest and have an honest compliment too, I can't see why you wouldn't want to let her know how you feel.

Just my two cents.