If you have monthly earnings above the level of substantial gainful activity, it is problematic for your disability or SSI benefits.
For 2008, the SGA amount is $940. Earnings over that monthly amount can present real challeges to receiving benefits. Depending upon the circumstances, you can be denied benefits (or the benefits you receive can be terminated) if your earnings are too high.
However, if your earnings are not way over SGA, there are ways to reduce the earnings counted by Social Security when determining whether or not you are working at the SGA level. These are called Impairment Related Work Expenses (IRWE).
Impairment Related Work Expenses are unreimbursed expenses that are 1) related to your impairment and 2) help you to work. Many different expenses qualify, but the most prevalent expenses are medical and medication costs. So if you are paying for medical visits, medication or for insurance copays, these are likely to be expenses that can reduce the earnings counted for SGA income. There are many more qualifying expenses. For more information, read Social Security Ruling 84-26.