You can go overboard with hat decorations. I prefer to have a clean profile with a punch of WOW on one side of the hat. It makes people watch you just to see you turn your head around to reveal a different look.
Here is the final step for making the hat above:
Tips: You can make inexpensive wool felt by buying wool coats from thrift stores and shrinking them in a wash/dry cycle. As for the hat form, my styrofoam head was very small, so you may have to shop for a head form that is 21-22 inches around (normal head size).
For more hat decorating ideas, see post DIY Millinery: Fascinators