Fugue ME!!!

The best way to improve counterpoint remains, in my opinion, to listen to and play as many of the compositions you wish to emulate as possible. It's also worth deciding whether you intend to compose away from the keyboard (as Bach did), or at it (as some more recent composers have done). Both have upsides and downsides, the former arguably substantially harder than the latter. I've found a mixture of both disciplines to be fairly rewarding; composing at the keyboard is an excellent way to improve one's skills improvising, and also at playing by ear.

Irrespective of which method you choose, there is no way to learn how to write fugue subjects besides playing and listening. The best work so brilliantly that the piece seems to flow from an ulterior source, some flop after the exposition, and some do not work at all. Over time, one can hear (to improving degrees of accuracy) from the first playing quite how profitable treating a subject might be.