Roll 3-inch strips of masking tape into a loop by connecting the ends, adhesive side out. Stick one side of the tape to the back of the pennant and the other side to the wall. Press flat with your hand to hold the pennant flag in place. This is not recommended for freshly painted walls. If the pennant flag is lightweight, you may choose to use painter's tape instead, which will lessen the chance that the tape will remove the paint on the wall's surface.
Tear a stick of poster putty into five to seven one-half-inch strips. Rub the putty between your thumb and forefinger to activate the glue. Press a piece of poster putty onto the back of all three corners of the pennant flag. Put the remaining putty pieces along the long edges of the pennant. Press into place on the wall's surface. This will hold felt and fabric pennants, but periodically add new putty for maximum hold.
Adhere the pennant to the wall using a staple gun or thumbtacks. The holes these items create are much less noticeable than those created with hammer and nails, but they are just as durable. When you want to remove the pennant, fill any visible holes with wall spackle or toothpaste. Before hanging the pennant flags, decide exactly where you want to place them to minimize the number of holes created. If possible, use existing holes to avoid making more.