This article is from the sci.lang.translation FAQ, by James Hester (firstname.lastname@example.org) with numerous contributions by others.
Methods that have worked in the past (contributions welcome) include:
(a) Engaging a debt collection agency
(b) Including a phrase in your invoice saying that discretionary charges
are levied on overdue amounts, followed by a reminder on the day that
the payment becomes overdue which states the extra charges
(c) If in the USA or UK, you might consider the Small Claims Courts in
which you needn't be represented by a lawyer.