I have found a great post on common printing mistakes by Pascal Smits about the most common printing mistakes to avoid. It is worth to read the entire post here is just a short summary of the main points:
1. Bad spelling and grammar
2. Poor font choice
3. Using CMYK color mode
4. Correct print resolution
5. Remove color from B&W images
6. Use a bleed
7. No .gif or .png
8. Use vector text
9. Print a proof
10. Give your design some serious thought.