It's true you need subjects and plans. Having tasks prepared that you can assign yourself is a great motivator, as long as you both a) force yourself to do them and b) credit yourself for progress (which can be as small as simply completing today's assigned task, even if you were totally dissatisfied with how it came out).
After enough time drawing every single day, your mind gets the idea that it's not going to stop. I don't really get artists block any more. I still feel bleh, or even despairing, at times about subjects, but that doesn't matter -- it doesn't stop me from doing the work, which is the most important thing.
One other thing that is important in making "doing the work" as approachable as possible, is to have a well defined overall framework for how an artwork is made -- what are the components, and how it progresses. I would recommend Vilppu's Drawing Manual for this purpose, it is very clear, divided into 12 stages (= 'week' of study, if you are taking it as an overall course to polish all your art skills.) that each build on earlier stages.
That last part is particularly important IMO because of the tendency, even among professional artists, to try to 'jump ahead' (skip preliminary steps like thumbnailing) -- which is satisfying when it works, but it's more common that it really doesn't work out, and that's discouraging. Having a reliable structure prevents you from getting in that situation of being 'in the middle of nowhere, out of your depth'