How can I improve my drawing?

Drawing is the foundation of all other types of art. So it's really important to have a strong grasp. It looks like you are into illustration and anime type drawing, but you still can benefit from taking a life drawing class at some point, which is with a nude model. It helps you to understand how the body move and the anatomy behind why they move that way. Your stuff is cool, stylistic.

Your drawing is good. Basics are there. Since you want to draw Manga try to get this done in PC. Like coloring. Fastest way to improve your skills is getting advice from other Manga Artists.

Post your drawings in Art galleries of MangaRaiders forum. Its a Friendly Manga Artists community. You will get tons of friendly advice and comments.

Practice every single day, and if youre' really into art, try using different mediums as well as much as possible. Purchase some books that explore the type things you like to look at. Emulation is the beginning of most ppl's style so don't be afraid that your work looks too much like anothers'. I'll jump the gun and suggest you check out a few books under the name Comikers, it showcases a lot of anime/manga artists and how they create their masterworks... enjoy!~ you can find them at most barnes and nobles/borders.