At the end of a workshop give each participant a postcard or greetings card with a stamp on it.
Ask them to each write a goal for themselves on the card. This should be something that they would like to achieve in three months' time (you can adjust the time according to the context).
They need to: put their names and addresses on the cards and hand them back to you.
You need to: post them out in 3 months so they can check in to see how they did with their goal.
This is a great way for your participants to check in on their goal and remind themselves what they wanted to achieve.