How much money can I earn and still collect SSDI benefits?

I often am asked the question how much can I earn and still keep Social Security Disability Insurance benefits?  (I am also asked the same question about SSI and will address it in another post.)
The Social Security Administration (“SSA”) wants people to work so they have set up various work incentives which allow people to work and collect SSDI at the same time.  For instance, the SSA has what they call a trial work period during which you can keep what you earn and still collect benefits.   You must report your work activity to the SSA and continue to have a disabling impairment.
The trial work period continues until you accumulate nine months (not necessarily consecutive) in which you perform “services” during a 60 month time period.  The SSA will consider your work to be “services” if you earn more than $670.00 a month in 2008.  This amount increases to $700.00 a month in 2009.
After your trial work period ends, your benefits will stop for the months during which the SSA considers your earnings to be substantial.  Substantial means that you have earned more than $940.00 a month during 2008.  For 2009, you will be able to earn $980.00 a month.  Different amounts apply to people if they are blind.  The monthly amount for people who are disabled due to blindness is $1570.00 a month in 2008.  For 2009, it will be $1640.00.
It is possible to obtain benefits again for an additional 36 months after completing the trial work period if your earnings fall below the “substantial” amount and you continue to have a disabling impairment.
To summarize, you can collect benefits for nine months regardless of your income.  After that period, you cannot earn more than substantial earnings.  Thus, you will only be able to retain benefits if your income is below $940.00 a month in 2008 or $980.00 a month in 2009. 
There are additional factors that may be considered in calculating those amounts.  For instance, you may be able to show that your earnings are actually less because you incur job related expenses that non-disabled people do not incur.  (I.e., because of your medial condition, you need to take a taxi to work instead of taking public transportation.)  For more information about working while receiving Social Security benefits, you can have a look at Social Security’s publication, “Working While Disabled – How we can help.”  http://www.socialsecurity.gov/pubs/10095.html