Thanks for your review! I'm glad you like it! You get better by drawing more and more along with the will of exceeding your own capabilities. Judging by your gallery, I can definitely see some progress in your work so just keep doing what you're doing and you'll get there in no time. The thing I did when I first got a tablet was find an artist I looked up to the most. When I was younger, I really enjoyed the artwork from Darkstalkers 3, Marvel Super Heroes vs Street Fighter and Marvel vs Capcom A LOT, so that was my starting point. I used to use those as, sort of, a reference (and still do, sometimes!) to find out what it was that made those artworks look so well in their simplest form. Sometimes, I would draw the characters as I saw them just to get an idea of how everything worked and try to understand what Bengus (the artist for these games, if I recall correctly) saw.
After a while, I ran into Kim Hyung-tae's artwork and fell absolutely in love with his style. I used that as a bar to reach and with each time I colored something, my goal was always to have something that resulted in something of his flavor (my Iishi Haruko piece [which is also my current avatar] would be the best example of homing in on his style). This led me into all sorts of areas of study in regards to shadows, color theory, harmony, soft vs hard shadows etc. I found it also good practice due to his intricate character designs and poses; a lot of other artist sort of follow the same generic base design, however, Kim's work is more elaborate and surreal, yet, visually appealing--that's a different discussion, though.
Once I became more aware of how it all works together and reached that bar, I started venturing out of that comfort zone and began to expand out into more art styles and approaches to certain situations in search of my own. Samus, here, is an example of me testing out the waters of portrait/semi-realism art as well as starting off in gray-scale with color overlays as opposed to working directly in color (I think that answers your latter question).
Of course, all of this worked for me so the same method may or may not work for you, and I still have much to learn myself so I'll refrain from making any blunt suggestions. Again, just keep doing whatever it is that you're doing; the progress is definitely apparent.
Also, I don't really have a sketchbook... thing... per se, but I have a deviantART with more of my work: