Have both a written contract and divide up your fee into 2 or 3 payments. One payment should always happen at the time of the first meeting (that would be the nonrefundable deposit), then have them pay the rest at the finish of the job. If the work goes on for a really long time then maybe add a 3rd payment somewhere in the middle of the job. Just always get a deposit up front and final payment at the time you hand over the files/prints preferably in person. Having all that spelled out in a written contract makes it easier for everyone and gives you protection too. So sorry you got taken advantage of by those folks.