Plug boards are a lot of fun and can get traffic flowing to your site from unexpected places.
Plugs are simply little tiny banners. They are typically 80x30. You can have one made for pretty inexpensive. Then you go to a plug board, type in the url for your graphic (plug), the url for your site and sometimes you get a chance to add in your site name. Click submit and your plug will show up at the top of the list.
The main rule of plug boards is to not put up the same plug or site before your previous plug is off the board.
You can view mine at Working WAHM and one geared for craft sites and blogs please at Little Scrapbook Shop. I have some other sites listed under my plug board so you can hit more. :)
