A useful trick:

Select some text in any program and then press Command-< (less than), to automatically look it up in Nisus Thesaurus.

Optionally, if you select text from within a text input field, you can hit Command-Shift-< to paste in the currently looked up word in Nisus Thesaurus.

Alternatively, if you don't think you will remember this, just visit the Services menu and the same operations can be done from there as well.