I personally think that nothing but trial and error can make a good solo. Firstly, it is helpful to know some basic theory. Know what key you are soloing in, and then mess around with modes, the thing with this plan is that so long as every note you hit is in key, your solo wont sound crap, its just up to you to figure out a melody and general "feel" for the solo. Just inmprovise when jamming and ultimately you will stumble across summin you like