For the salary field on an online app, I usually put a zero. Do you recommend that?

There are two salary questions on an online application.  If you can put "N/A" in either field, please do that. However, unfortunately, some of these fields require a number. 

1. For your previous salary question (which is increasingly illegal to ask, state by state), simply put your total compensation number from the prior year. 
2. For your future or desired compensation, do your homework on Glassdoor.com and Payscale.com and put the median compensation for your target role in your city.