always discuss salary face to face.

oops, I didnt see the rest of the question. sorry.

some applications will have a spot for preferred starting wage, so you could just add somewhere on your resume saying your preferred starting wage is so much, but i would also put negotiable, like you mentioned.

In my experience salary is one of the last things to be discussed at an interview, however, whoever is running this ad on Craigslist is probably asking for salary requirements in order to screen out candidates whose salary expectations are out of line with amounts budgeted for the job.
I would absolutely NOT include salary on a resume. You could include it on the cover letter or on a separate insert with your resume and cover letter, assuming this is going through the mail. It might be better to include a salary history showing some rapid salary growth as opposed to stating absolutely that you want $40,000 or $60,000 and thereby maybe closing yourself out of a job interview. But if it says that you are earning $xx,000 and that is 15% better than your last job you are conveying to the interviewer or hiring person that you have experienced strong salary growth recently.

In every case, state that salary is negotiable even if you include a salary history. Far-fetched as it may seem you might take less money for a good career move, not a whole lot less money but a little less money.

The only other thing I would add that the most important thing is to get the face-to-face interview, regardless of what the job pays. It seems to me that the hardest thing to do in this day and age is to get yourself in front of someone who is doing the actual hiring.

Last year I placed a listing on Monster for a job at our affiliate in Toronto and I got over 200 email responses and resumes and frankly once I found ten likely candidates, after about going through 75 or so, I stopped reading. That is the reality of hiring nowadays and a professional head-hunter is going to go through the same process. The hardest part of the job is separating the wheat from the chaff.

SRM

There are good answers here, but practically speaking you should just include a number in your cover letter. And it should probably be firm. No reason to say you’re willing to negotiate if you’re not.

Anybody who’s asking for your salary requirements upfront like that is probably working with a fixed number in their budget, and they want to weed out the people who are overqualified and overpriced.