You can occasionally give small cash gifts which are used to supplement the basics. My brother is in this situation, and my mom often gives him a couple hundred bucks every month or two to supplement disability income -- but keeping him under the Medicaid and SSI income/assets thresholds.
In terms of estate planning for such a person, a special needs trust is also an option.
"Hey, for every ten dollars, that's another hour that I have to be in the work place. That's an hour of my life. And my life is a very finite thing. I have only 'x' number of hours left before I'm dead. So how do I want to use these hours of my life? Do I want to use them just spending it on more crap and more stuff, or do I want to start getting a handle on it and using my life more intelligently?" -- Joe Dominguez (1938 - 1997)