Short version:
Years of practice and keeping on drawing.

Long version:
I've been drawing since I was a kid, and eventually moved from pens and pencils to digital art. This is about 20+ years of drawing experience under my belt *accidentally shows age*. Though I think only recently in the last couple of years did I actually get somewhere where I'm feeling truely pleased with my art.

If you really want to get somewhere and improve your skills, make using references your best friend. I actually regret not using refs for my art when I was in my teens, I had this mindset of "I should but meh." And looking back on it now I really regret it because I really wonder what my art/skills would be like now if I'd only used refs early on.
It's only when I started using references and tutorials that I felt like my art truly began to improve.
I generally look at this tumblr here for various art tutorials and stuff. DeviantArt isn't too bad for looking up tutorials too.

Inspiration wise for me it varies a lot. Usually I'll end up drawing whichever character has a hold of my art muse going at the time, whether it be from rp things (both serious or not) or completely random inspirations from things I've seen or read about during the day/night.
Rping helps a huge deal I've found, it's like inspiration fuel, and the scenes are right there for you to take and draw.

Also getting into art slumps is pretty normal, every now and then I'll be drawing or looking through my drawing folders and feel like my art isn't getting anywhere, when this happens I try to think of things I can improve on, or how can I improve *thing*. Sometimes I'll even go and compare my old art with new art just to prove to myself that I've improved over the months/years.
It's also important to remember that improvement isn't immediate. You don't instantly pick up or master an art skill, it takes time.

I think that's it, its 2am LOL

Are you having a hard time? Everyone makes mistakes and can get stuck. When you do get stuck stop for awhile and relax, maybe watch some TV and think about on how to get unstuck. If you want to get better keep practicing, use references if you have too, and keep trying if something will not work. How did I start art? I started doing art almost last year, before I could not draw a thing but I noticed im getting better than when I first started. I am known as Link, mostly because its one of my ultimate childhood characters and I am very well known with that name on the internet. My reputation has grown a lot and it happened just by doing what I do! I am almost done with my 2nd TEF drawing which somehow looks amazing to me once more, that's how I know that I am getting better. My character, I designed it with certain things I like and qualities I feel it should have, Ace drew it how I exactly saw it!

Practice makes perfect, it may not sound true but I feel it has been working for me. Good luck though and keep trying!