The 6-3-5 method is a good way to come up with a good quantity of ideas in a short time. It stands for "In a group of 6, each one writes 3 ideas every 5 minutes".
To do so, you need to hand out papers with a table of 3 columns (1 per idea), 6 rows (1 per participant) and a clear sentence defining the problem on top of it. The facilitator must take care of the timing and tell everyone to pass their paper to their right every 5 minutes. When they get a new paper, participants should first read the previous ideas and then start writing the 3 ones they can think about.
When the 6 rows are full, it's time to discuss, remove the redundant ideas and group the similar ones.
As usual, before starting to use this method, all the participants should be aware of the brainstorming rules and that it's ok if they can't fulfill the 3 columns every time. One tricky part of this method is the fast pace which can stress the participants and block them. But I think that being able to see all the ideas the others wrote already can actually help.