As many of you know, I spent a quarter/semester in Washington DC last Fall. I worked for NASA Goddard Space Flight Center.
I had a friend who wanted to know a good timeline for officially applying for everything. So here's my timeline of events for large "life" events:
Fall 2010: Decision to take a quarter off in Fall 2011 decided (Washington DC or Co-op elsewhere)
Late Fall 2010/ Early Winter 2011: applied for Washington Program
Late Winter 2011: Acceptance into the Washington Program
Spring through Summer 2011: Applying for jobs/internships in the DC area
Fall 2011: Worked in Washington DC
So, you should plan for about a year in advanced. However, there are other programs and events that take longer. If you are interested in a specific program, it doesn't hurt to look ahead of time.
Also, I know many of you are rushing to graduate. But just a reminder: it isn't a good idea to graduate if you don't have any job experience. If you can, I would recommend two engineering internships before you graduate. If you can't, one engineering internship is good, but two is better. And if you absolutely cannot land an engineering internship, make sure to have a job. I've seen people who don't have any experience whatsoever (no volunteer, no McDonalds, nothing).
In addition, I would be wary of Summer school. Working and attending school is often a good setup, but not always feasible. Use the summer to build your skills, even if it is flipping burgers. You'll be able to market your skills for punctuality, teamwork, handling money (trust), dealing with customers professionally and politely. It's way more than school will ever give you.