Just posting a couple of tips that have been incredibly useful for myself :D They apply to digital art, although i suppose you could modify the drawing tip for use with real media by using a mirror.

• While drawing, try to flip the image horizontally from time to time. (So if you're drawing a person facing left, flip the image/canvas so they face right.)
  
  A lot of the time you'll find that there are things about the flipped image that look a bit off, even though it probably looked fine unflipped. This is probably the singularly most useful trick i've discovered, and very helpful if you don't have a friend around to critique your work. i only wish i'd learnt to do this years ago ^^;
  
  This is obviously easiest to do for digital work. If you're working on paper/canvas, you can achieve a similar effect by holding it up to a mirror.
  
  
• A problem i often have with my colouring is low contrast, which makes the final image look a bit weak. Something i've started trying to do lately while CGing is to create a new layer right at the top of all the other layers, fill it with grey, and change the colour mode to Hue.
  
  While this layer is visible, it ignores all the hues from the image underneath, so all you see are the values (how light or dark something is), as if it was a black and white photo. If an image has an attractive distribution of values, it's more likely to look good with colour as well. (Which as a sidenote is probably why many artists use the technique of underpainting)
  
  You could also check the values by desaturating the image every now and then, but i just find using this layer method more convenient--one click to turn it on, one to turn it off :)