Most (all?) of us selling services have clients that went missing after doing the work. This has happened to me (when I started, I’m not so naive anymore, and luckily it was only an invoice of 200 USD) but I was not aware the problem was so common. I just discovered that the site the World’s longest invoice claims that 15M USD (and growing) have been stiffed by clients.
There are different strategies to avoid this situation in the first place (once it happens and unless it’s really a considerable amount of money, it’s simply not worth pursuing them ). Sure, the obvious one is asking the client to pay the full amount in advance but this can scare off the client (who is also thinking you may run away with the money before doing the job if s/he pays you upfront). Having a nice looking site with testimonials will help to inspire some trust in you but this may not be enough for reluctant clients.
What has worked for me is offering a free test migration . The client gets a partial but complete site migration (i.e. the migration only contains part of the posts,pages,… of the original site but those that are included are fully migrated) which allows him to test and validate the quality of the work before paying but that, alone, is completely useless. Then, if he is happy, he pays the full amount (now he already trusts me) and we go ahead. If not, we just stop there (and yes, the work done so far goes to the trash but at least this way I’m sure that once the client pays he will be happy with the result; this is a different discussion topic but I prefer this than arguing and/or refunding clients that didn’t read the the conditions of the service and had unrealistic expectations about it).
Obviously, I’m not suggesting this is the best strategy nor that it can be applied to the product/service you’re offering but I’d love to know how you deal with this problem!