Leave space on the walls. Spread out fewer decorations rather than crowding more posters and banners close together. Children will quickly be over-stimulated by a crowded, overly decorated wall and will be less likely to grasp what is there.
Hang educational posters and signs low on the wall. Realize that the children in your class are not very large. Don't make them strain their necks to see what you hope to contribute to their learning.
Involve the children. Let them help you hang posters or choose which wall hanging goes up next. This will keep them more personally engaged in the decorations once they are hung.
Display student art to give them a sense of involvement and keep the classroom cheerful. Hang an "artwork" bulletin board before students arrive for the year. This will prevent you from having to leave a large section of wall blank.