Visualising ideas will make them easier to understand and modify, and will in turn stimulate new ideas.
How
Get people who can draw to sketch ideas during a group brainstorm.
The drawings don't need to be perfect: they only need to have just enough detail to communicate the idea.
Visuals are then shown to the group and used to stimulate discussion and more ideas. The visualisers themselves are most effective when they are part of a team which also has a facilitator.
Resources
Someone who can draw, paper, marker pens