The best way is to speak about your desired salary in ranges. For example, if you really want $50,000 a year, tell them that based on your experience and skills, and the current market rates for similar positions, you are looking for a salary of $48,000 to $55,000 a year and that you are flexible when it comes to salary levels.
Your salary request must be reasonable, within the typical range for that type of job, and you must let the employer know that you are flexible. They just want to know that you are in the same ballpark as they are. Salary.com and PayScale are two websites where you can research salary ranges for your target jobs.
Of course, the salary questions can come up mid-interview, so be prepared.