There is a ton of solid advice on this thread. Do some studies of skulls, use references, vary line weight (maybe try using pencil if you've been using ink? It's easier with pencil I've found). Make a bunch of bad stuff without beating yourself up about it, everyone makes bad art constantly except for that one person we're all jealous of. Just no one is broadcasting it.
Also when adding details keep your wrist as loose as you can, even drawing more from the elbow, and try and suggest lines instead of connecting them, or construct them with shading instead of lines. @Hodge is really good at this, notice how in the semi-realism and realism around the lips; they aren't drawing every line that defines the lips, they are leaving a gap where they would connect and letting your brain fill in the rest, and instead of lines for the colour to the lips they're using shading. Be very careful about drawing in under eye bags, and that crease coming down from your nose to the corners of your lips, those age people pretty fast if you use to hard of a line. @jay3spin did this in the middle picture by drawing just a hint of cheekbone, anything more would have made them look too gaunt.
Also like...I have no idea what stage you are at but when you're drawing eyes if you shade the upper third dark connecting into the pupil and do a gradient down the iris so the lightest part is under the pupil in a bit of a kidney bean shape like hodge did here, people go wild for that shit.
Copy work you think is cool. Like don't post it and call it your own, but everyone is copying and learning from everyone else constantly. Keep drawing all the time. When you feel comfortable enough post your work for critique.