1) Don't reinvent the wheel for the sake of being different: There's a reason for most design rules beyond taste and opinion. If something is tried and true, and it works, don't change it without a very good reason.
2) Start with good idea: Polishing a bad design will only go so far. It takes dozens bad ideas to come up with a one good one. Don't fall in love with your design only to have to justify it to your client. Be willing to throw ideas out and restart.
3) Brainstorm your ideas: Share your initial designs ideas. Even if you don't use any of the input, just talking about the options and explaining your work will spark new directions for you. Know your own feelings, the reasons why you did something, about the design and be clear with yourself about what works for you and what doesn't.
4) Make sure to design for your target audience: This is so obvious, and yet, so many times I have seen inappropriate design elements, colors and fonts in a design targeted for a specific group. The design for a concert poster for children's play will have to use a very different approach if it's target audience is the children the if it's their parents.
5) Stop using too many typefaces: A typeface (also known as font family) is a set of one or more fonts each composed of glyphs that share common design features. Each font of a typeface has a specific weight, style, condensation, width, slant, italicization, ornamentation, and designer or foundry (and formerly size, in metal fonts).
6) The beauty or the power of a good design is often in its simplicity: If you don't have a very good reason to use a certain font, a shape or a certain color to be in your design, delete it. Don't fill in the space in your design with "stuff" just to make it "interesting".
7) Use the "white" space to your advantage: The empty space in a page layout (or any graphic design) is called the white space. The balance of a good design often depends on the relationship between the graphic (or typographical) elements and the white space around them. As a matter of fact, you can give power to an element in your design with the white space associated with it. White space is not empty space!
Remember that the quality of your graphic design is in details. Be diligent in your decision-making - there is a difference between Arial and Helvetica; and you should know why you choose one over the other. Choose the right color for the right reasons, not because it's pretty. Different colors have their meaning.