You should use simple graphics that can visually depict the theme of your event right away. In the example below, I placed some spider webs and spooky eyes at the top above the event title, with a ghost predominately displayed below it so that, even at just a glance, you get the idea that there is some sort of a spooky or Halloween related event that the flyer is telling you about. You can use clip art or even insert photos of previous events, your school, team, etc.
Borders can work in place of main graphics when you have a lot of information that you need to convey. With a related border, such as falling leaves for a fall festival, or pom poms for cheerleader tryouts you can still convey a sense of what the event is while allowing room for the extra information that you may need to include.
Be careful not to over clutter the flyer. Using too many graphics will make your flyer appear jumbled and the information that people need to know will get lost. At the same time; however, be careful that you do not have too many large blank spaces because then there is nothing to draw someone's attention to your flyer. You can use a small filler, like the bat that I used to close up the empty space at the bottom of the flyer; or you can simply increase the size of the existing content to make everything proportional to the size of the flyer.