Use bright contrasting colors like this
-it wears on the eyes

Use alternating Capital and Lowercase letters through out a whole page like this
-it gets old really fast

Fill up your main page with tons of banners, pictures and buttons
--it takes waaayyyyy too long to download and may look cluttered

DO...

Use basic colors with either white or black backgrounds
-it makes your job easier, things tend to clash less, text is easier to read

Use techniques like adding special symbols