[anto1ne]

even faster summary: install yslow and PageSpeed, follow recommendations.

I've seen enormous improvements on site which didn't follow any of this rules, something like load time divided by 2 or more, but of course it depends how bad is the site you start with.

[dunkjmcd]

Those plugins are great but you really need to look at the waterfall breakdown of your site to really grasp where best to spend your time. The net panel in firebug can be amazingly helpful at showing you where blocking and latency is causing the most grief.