It's always case specific, but it usually involves the manager supporting them by providing time and/or space. rubicon33's example and response is a great one. Put them in a situation where their strengths can be well utilized and the effect of their weaknesses minimized.

Also, not everybody has the personality or energy to constantly stand up for themselves. A lot of people will eventually crumble or leave. Don't make it so the only way the employee can do their best (e.g. what rubicon33 describes) is if they put their foot down and scream.