Honesty. The first and foremost thing to always keep in mind is honesty. When critiquing someone's project, you must use your most sincere and honest view. You can do this by saying exactly what you feel is working and what needs work. Honesty in this case is always the best policy.
Point out what is going right. The best type of critique is one that acknowledges the good that is working. Always look for the good that is coming out first, and when you do you will be in a better position to get to the root of what needs work. Make an honest effort at this.
Constant improvement. The last step is to always aim for constant improvement, continue to move forward with what needs work and actually allow the person to be accountable to really get this done. Improvement doesn't happen overnight, so be consistent with what you see needs work for the person and continue to push forward.