For those who are already receiving Social Security disability benefits, Geri Kahn's California Social Security Lawyer blog has a well written post answering the question "How much money can I earn and still collect SSDI benefits?"
I recommend that you read her post, which weaves together trial work, the 36-month reentitlement period, and impairment-related work expenes. Further, as Geri notes, the SSA's website has an electronic booklet with more information about working while receiving Social Security benefits.
Geri Kahn is a Northern California Social Security disability attorney, with offices in San Francisco and Benicia.