No matter what your disability, the most important element of an application for SSDI/SSI is medical records and reports. The doctors who treat your condition regularly are the best source of information regarding your particular condition, symptoms, side effects, medications, treatment options and progress.
In particular, your doctor's opinion regarding your ability to perform work related activities carries the greatest amount of weight in a review of your application. It is important to note however that a letter from your doctor claiming that you are disabled is not enough to get an award of disability. A statement to this effect is essentially useless and meaningless to your claim.
The more persuasive documentation is a Residual Functional Capacity report. Social Security has made the completion of this report easier on claimants and physicians by producing separate forms for physical or mental disabilities that the doctors can fill out and return. These forms will give your doctor an opportunity to issue an opinion of your limitations from your conditions that prevent you from working. In addition to checking the boxes on the form, it is best if your doctor provides additional explanation or support for his opinions. Citing support for his findings will assist Social Security in reviewing the RFC capacities assigned to you.
As mentioned above, there are separate forms for physical or mental disabilities. If you are alleging that you are disabled by both a physical and mental condition (for example failed back syndrome and depression), both your physician and psychiatrist should complete the forms.
Finally, these forms can be especially important because when Social Security sends your file for a medical review, the reviewing doctor submits his own RFC form regarding your ability. If Social Security's reviewing physicians are the only ones to assess your ability to perform work related activities, the fight for benefits becomes that much harder.