With more and more applications online, the previous standard of putting “negotiable” in this field becomes less of an option as the form typically looks for a specific number or range.
The best advice you can get will depend on the application. If it’s paper application and you can write “negotiable” or something similar and feel comfortable doing so, go for it.
If the application give you the option for a range, determining that range will take some research, which you should do prior to the interview anyway. Find out the average income in the city you’re applying in. You can find that on this City Data site and other similar sites. Once you know the average income, find the cost of living for the area you are applying. The Bankrate.com cost of living calculator is good. As is this one from CNN Money. Run a few and get a good ball park range. Add in your fixed expenses and you’ll have a good number to start with. Then research what the typical salary range is for the position. You can find this information through a professional organization, simple Google search, Monster.com and other sources. Use more than one.
The above process will also help you with the forms that require a more exact number. If you are lucky the form will say minimum required salary, which means they won’t consider you if your number is above what they are looking for and you won’t consider them if they intend to offer less. This is a way of weeding out the applicants.
In the end, you will have to find what works for you and your life and lifestyle (hopefully it isn’t extravagant!). Try a few techniques, using your best judgement, you’ll eventually find one that works for you.
This Career Capitalist post gives a good conversation tree for how to approach this question in an interview.
This post from Quarter Life Finances has several pieces of good advice when dealing with online applications.