Besides what's already been said, my suggestion is something really simple that causes some people's writing ability to often improve - don't ever let yourself lax on your spelling and grammar. It's a personal pride thing for me, so I do it naturally instead of having to force myself (Heil grammar Nazi), but I always try my best to spell correctly in messengers, text messages, whatever. That way you're not tempted to fuck up your syntax consciously or unconsciously when it counts.
If your sentences don't make sense sometimes, speak while you type. Even if you just mumble to yourself, you're more likely to catch a ridiculous sentence if you hear it out loud, or let your brain process it more thoroughly (mumbling) than if you're just typing. Shit that makes sense in your head doesn't always make sense on paper.
If you hear a word you've never heard before, look it up, always. Try to use it in conversation, so it feels more natural, even if it's just "I'm feeling pragmatic today."
Instead of writing short stories, try writing the first chapter for a novel. Even if it's just one chapter that's not linked to anything and never goes anywhere, making one single idea last that long really helps when you need to write descriptively later.
Yeah, that's all I can think of that hasn't been said already, though reemphasizing the 'read a lot' point is also a good idea.