taking the previous idea one step further, if you are playing a fugue, particularly a 3 or more part fugue, play it with a voice omitted, which you hum/sing instead. Its important to keep the same fingering as you would have used if playing the full piece, even if this results in some odd fingering.
You can then switch to singing a different voice, each one in turn. This is really useful for bringing out the voices, and ensuring that the voices keep their own phrasing and dynamics, even when another voice goes against them. And even on the harpsichord or organ, although different from the piano, there are still ways of bringing out voices (different organ stops, articulation etc). Otherwise a fugue just becomes block chords, mashing up the dynamics of each voice.