Banners are what makes me decide whether or not to play the game. A good banner makes me think of a good webmaster. Here are some tips which you can use to enhance your banners and to fool more people into joining your game (only joking!)
Appropriate size - Too big will load forever and take up too much space. Too small ... what's the point of having a banner if you cant see it??
Colour scheme - should fit with your background and text
Pictures - Should be relevant
Avoid big white spaces which could be filled with pictures - if you have white spaces cut down the size of your banner
Writing - should be large so it can be read. And kept to a minimum - the game name and slogan nothing else. Otherwise it becomes cramped
Creativity - Play on words. IF you are called Stars Hollow have a dark banner with a black horse with a star in it's eye, or in the sky, or on it's rump... the possibilities are endless
Most importantly - Your banner sells your game. Make it fun
and inviting and interesting to look at.