Try it.
Songs
Posters
Economic reasons (money)
Glamour (uniform, bravery etc.)
German brutality (propaganda, eg. gorilla)
White feathers (handed out by women to symbolise guilt and shame of not enlisting)
Travel (adventure)
Money (fed regularly)
Women (popularity with heroes)
Guilt (not signing up)
Religion (god ensure survival)
Pals batallions (fought with friends)
Patriotism (King and country)
Fatherly instincts (protecting future children)