One thing that always motivates me is thinking about the amount of time the audience is investing in you. If you're speaking to 100 people, that's a solid 100 hour investment. This helps me put the time I need for preparation in perspective. It may seem crazy to spent hours and days to prepare a one hour talk, but it's not really not if you think about it like this.

Some more thoughts I've written on preparing a technical talk, I think complementary to the advice of OP: http://branchandbound.net/blog/conferences/2012/06/preparing...