Use part of a retrospective, daily scrum or other session to make a brainstorming session on ideas for celebrating.
Use for instance Brainwriting technique to generate numerous ideas.
Make sure ideas are written on a piece of paper.
Put ideas in a jar in the office, for later inspiration on celebration.
To boost the actual use, you can select a few ideas on the spot, and remember to return to it later on.
Additional effect: Doing activity will usually raise the awareness of celebration, and have a positive effect on team’s own initiative.