Here are some quick tips for providing useful critiques to your fellow artists:

• Be honest but respectful
  • Avoid being judgemental or saying something that would trigger an emotional response. Be factual
     
• Keep it short
  • Don't overwhelm the artist with a laundry list of nit picky things to fix.
  • If there are lots of areas to be improved, try focusing on one overall thing to work on.

• Ideas for things to assess
  • Proportion
  • Anatomy
  • Color
  • Composition
  • Perspective
     
• Say something positive!
  • It's easy to get discouraged after hearing something negative. And knowing what we're doing right is almost as useful as knowing what we could do better.
  • You could, for example, say how the work reminded you of an experience or had an emotional impact on you.

Did you think of something else that wasn't listed? Please let me know so I can update this guide.

Additional resources:

• Crash course on the art of constructive critique
• How to give an art critique