Keeping track of who owes you money will reduce a lot of headaches in the future. Create a bill of sale for each work you sell. Make sure you use a contract for every agreement you enter into.
If you make special payment plans for certain individuals, make sure you include that payment plan in the bill of sale. You should always retain the work until the final payment is made.
Always get a list of the work located at a dealer, a gallery, or an art consultant. Keep in touch with them regarding sales. Know where you work is.