I haven't been drawing much lately, but when I do draw digitally, I use a mouse because I can't get a graphics tablet yet.
I don't know how familiar you are with image editing software so bear in mind that some tips may require familiarity with them.
1) As with many things, practice plays a big part.
2) Some programs have a "stabiliser" or "smooth stroke" that smooths your lines so they are less wobbly.
3) If you're going for a "painted" style, don't worry about how "neat" it looks, especially when it's zoomed in. Trust me, it'll look just fine when you zoom out. Also, use a relatively large canvas.
4) If you want to do line art, use a vector graphics editor!! e.g. inkscape. You can easily edit the lines as opposed to a raster graphics editor e.g. photoshop.
5) Do a very, very rough sketch or outline on a separate layer beforehand.
6) If you're colouring in line art, fill in the area you want to colour with a single colour on a separate transparent layer and lock the transparency so you can't go over the lines.
It's probably still more time consuming than if you just use a graphics tablet and draw by hand (though idk because I can't compare) but it does make mouse-drawing a hell lot easier.
Edit:
7) Play around with brush settings and brushes! Because you don't have pressure sesitivity and velocity is kinda hard to do, you need to make use of the other stuff.
I can't stress how important using a vector graphics editor is. It's surprising how much easier it is to do line art with it (using a mouse anyway). Additionally, don't spend too long on a small area. If you think it's good enough, leave it.Don't waste time trying to get it "perfect".