Recently I was working on optimization of a site, and now finally its average page load time time has been reduced to 1.3 sec (source: Google webmaster tool).
There were a lot of ups and downs in page load time as few things failed and few worked exceptionally good.
I will rate these as top 3 techniques which helped
Reduce http requests (Use image sprites, combine css and js etc)
Use browser cache properly
[*]Gzip your html, css and optimize images
You can also see the test results for the site on http://www.webpagetest.org/result/100125_4JEY/