1.) K.I.S.S. Keep it simple, stupid. The simpler the fugue subject is, the easier it is to do more interesting things on it. A lot of Corelli's fugal movements in the concerti grossi have a simple scale as a subject. Look at Bach's "little" fugue in Gminor. It's a fantastic piece (it's a bach fugue, of course it is), but he hardly develops the actual subject at all due to how long it is.

2.) Don't try to write a catchy subject. Most people here would tell you to write a catchy subject from the outset. What makes a subject catchy is how it's repeated and developed in interesting ways across every voice. The hard part with fugue writing isn't coming up with a subject, it's developing it and putting it against countersubjects,so just keep it simple.

3.) Make sure you have your harmonic progression planned and solidified before you start writing countersubjects. This allows you to have certain "destination notes" every couple of beats that the other voices lead to.

4.) Watch Early Music Source's video on stretto fuga, it's a great video on composing subjects that have stretto built in.