The 100 thumbnails exercise is something I got from a professor. She would urge us to keep trying ideas out on a particular challenge, even if we felt we had an adequate solution. The first twenty or so tended to be obvious, fine solutions. The next few tended to be terrible. About halfway, it became very difficult to think of new ideas. If we kept going, we would eventually break through and discover a whole new way of thinking about the problem. If I remember correctly, the TV show Mad Men often depicted copywriters being told to come up with a vast number of taglines (probably for similar reasons).
Rapid sketching is something a few of my bosses have encouraged. On consulting projects we sometimes do it with clients. Google has Crazy 8's. The basic idea here is to set a time limit that makes it virtually impossible to draw patiently, encouraging participants to get any and all ideas down as quickly as possible. It forces you to think about the bigger picture and step away from small details.