White Space is a very good thing - it gives room to view and appreciate a good design.

Bold and Italics are for emphasis.
If you emhasize everything, nothing is important.

ALL CAPS in print is the same as screaming.
DO YOU LIKE TO BE SCREAMED AT?

Work with what you like. Use colors and images that appeal to you.
What do you like?

Use appropriate colors and images for your project.

Do not clutter your project with too much activity.
Only include what needs to be communicated, use your design and placement to emphasis key points.

Do not use too many fonts. It adds CLutter.

Think about your audience. You would not use the same colors to market to professionals as you would to children.

Good writing is important. Spelling and grammer are important. Take the time to proof read your marketing materials and find someone else to proof read it too! People do notice when you mispill words. You did notice right?

There is a place and time for everything though.
If you really think you need it, try it with and without it and see what gets your point across in the more effectively.

...and most importantly have fun with it!