[Tex/LaTex] How to get bold math symbols

boldfontsmath-modesymbols

To make Latin-letter variables bold I can use e.g. \mathbf{a}, but while putting Greek letters or symbols such as \nabla inside \mathbf doesn't cause any errors or warnings, it also doesn't do anything else.

What is the best way to make bold math symbols, in particular Greek letters and \nabla?

The AMS Short Math Guide recommends the \boldsymbol and \pmb commands (and suggests that you use the bm package for the former to get a more powerful version than provided by amsmath).