It's true--mirrors are the bomb. I might also recommend using a digital camera! This is my favorite method of attack because I can take a picture of the pose I want (especially when needing things like hand references) and upload it to my computer to be put in the background. You can also ask people around you to pose! I've done this before, and besides the odd looks, nothing too bad came of it.
I don't have any sarcasm for you, but I AM more than happy to give you some criticism and lots of suggestions! ^__^
There are, as others have mentioned, some problems with anatomy, the general figure, etc. I have found that by far the two easiest ways to fix these are as follows:
and 2. Draw from life!
I can't really give you tips on the first, but I CAN give you tips on the second.
The key to drawing from life is to remember that PHOTOGRAPHS ARE YOUR FRIEND! My favorite source is deviantart stock art--in fact, I sometimes find stock photos to draw from before I even know what I'm drawing.
If you're not familiar with finding things on deviantart, I'll show you ^__^
The pose in this drawing reminded me a little of something from dragon ball Z, however, I decided to do a more vague search first for something similar. I went to deviantart and did a search for "battle pose", then narrowed the search down to resources, then down to stock images. However, since this didn't return anything like I wanted, I chose to go with my first hunch--DBZ.
Staying within the Stock Images category, I ran a search for "dragon ball" and after sorting through a bunch of pictures of balls, I came across some pose pictures closer to what I was looking for!
This image was returned! ^__^ Though the pose exactly does not match the one you used, the poses used are similar, and you can use individual parts for reference--or even change the original pose a bit!
There are lots of stock and pose resources out there, so don't be afraid to use them!
The other thing I suggest, if you are not doing so already, is to draw from a base. I know it's a pain in the butt and it sucks and it's hard to get used to, but DO IT. I swear it helps (trust me, I went through the phase where I told everyone it was just my "style" not to do that---and was snapped into reality when Steve Bennett informed a room full of artists that that was, in short, just an excuse. Even more shocking was that the excuse he worded sounded exactly like mine! ^.~).
I drew a sort of example skeleton on top of your drawing, to show you what I mean. The base doesn't have to be detailed--a stick figure like I drew will work just fine.
A couple things are pretty apparent with the help of the stick figure skeleton:
1. There isn't, by far, enough neck
2. The bottom portion of the arms are too long
3. The hips are not wide enough (Or rather, don't really curve at a normal angle)
Stick figure skeletons are a GREAT way to work out problems like this WITHOUT having to redraw half the image, which sucks.
It does take a while to get used to them, however. One of the things that helped me to practice with them was to, using paint, draw a stick figure over my reference picture! This gives you a sort of reference for you base! ^__^
The other thing that really stands out to me is the lack of attention detail in the image. Lots of small details are missing and others are sorely neglected. Detail can really make a picture, so be sure to give lots of time to it! When drawing jeans, don't be afraid to draw in things like seams, rivets, tears, etc. ESPECIALLY don't forget clothing folds! Other details, like the buttons on his shirt, and his shirt collar, need to have a bit more thought put into them. For example, in the image, the buttons are crammed close to the end of the shirt, not all the same space a part, and not even quite all the same size.
I can't really explain how to do clothing folds, however, the "Manga University" has a WONDERFUL tutorial on them.
You can access it here by clicking "clothing and folds":
So yeah... Now that I've talked way to much and given you far more information than you'll ever need, or probably use, I'm off. o.o Hopefully, if you can bear to read my novel-of-a-message, I was able to be of some help! ^__^