It's all too common I'm afraid, and a reason why sites like upwork.com or freelancer.com do so well as they handle the admin. Now there is also utopian-io (check the hashtag here on steemit), blockchain based model where there is a record of work with no dispute.
However, general advice:

• Don't undervalue/under-charge your service. Others are cheaper only because they are inferior. People will assume you are desperate and take advantage. Charge higher, get genuine clients.
• Get payment upfront for milestones. Minimum 30% before commencing.