You can use TfL's getaheadofthegames.com website to find out where to avoid and how to plan your journeys around the Games, at the 14 London venues, and the other 11 UK-wide locations.
You can use the website to find out whether stations or lines that you plan to use will be amongst those most affected, with predicted day-by-day impact, along with advice on your options. Additionally, this "hotspot" detail now covers Tube, DLR, London Overground and National Rail and is available for both the Olympic and Paralympic Games.
It may be interesting to bear the website in mind when reading Daniel Ruiz's 2012 series of articles on the Olympic Games transport legacy (read article 1 here and article 2 here)
from the website....
Top tips for travel during the Olympic and Paralympic Games 2012:
Find out if your journey will be affected and plan your travel in advance
Consider alternatives - another type of transport might be a better option for you
If you aren't going to a Games event, try to avoid venue stations, particularly at the busiest times
If you need to travel, look at re-timing so you avoid the busiest times
Avoid driving in affected areas if possible.
If you need to make a car journey, be prepared for delays.