Here you have general rules but I think that this guide will be very useful specially for Media Temple GS users (like me).
As maybe you know when you buy this service you can use each month 1000 GPUs (which stands for Grid Performance Unit).
According to Media Temple website “1 GPU = 7.24% of 1 CPU for 1 hour. You can use up to 1.38 GPU’s per hour and not go over the 1000 GPU limit”.
After this limit you have to pay $0.10 per GPU.

If your WordPress is not correctly configured you’ll need a lot of GPUs for few thousands of users.
After a good optimization instead you’ll be able to respect the montly limit without problems (if not, maybe you can consider  to upgrade your plan to a dedicated-virtual server).

I read a lot of different opinions about the Media Temple’s Grid-Service.
Personally i love it (keep up the good work, guys!), but i’m absolutely sure that you have to work a bit with your theme to take all the advantages of this technology.

So, roll up your sleeves, and it’s time to start.

First of all, backup everything, then…

First Step – Optimize

1) Upgrade to latest WordPress release

2) Delete all the unused or unwanted plugins (this is the first reason why you need 10-12 seconds to load your homepage).

3) Clean up your css code using a css compressor like styleneat, cleancss or csstidyonline.

4) Reduce the PHP and database queries.
For example, on your header, use absolute URLs (http://mysiteblabla.com/wp-content/themes/example/style.css instead of <?php bloginfo(‘stylesheet_url’); ?>).

Second Step – Install some plugins

After that you can install 6 “magic” plugins:

5) WP Super-Cache (to cache all the dynamic requests as static html files).

6) WP Tuner (to analyze your database and to find slow queries or plugins).

7) WP Smush.it (to reduce image file sizes using Smush.it)

8) WP-DBManager (again, to optimize and repair the database)

9) Disable Revisions and Autosave (every time you change a post you create new row in the database, you don’t need this feature).

10) Clean Options (to find orphaned options in the wp_options table)

Third Step – Finish up

11) Use Amazon S3 to serve static files (useful to save bandwidth and cpu). If you need help follow this guide.

12) Be sure to have the favicon.ico file. Otherwise WordPress will generate an “invisible” 404 error (and a large amount of server load)

13) Disable Hotlinking of image. Two tutorials from jaypeeonline.net:

14) Keep RSS ‘content thieves’ away (tutorial here). And maybe use feedburner without any extra plugin to redirect your users.

15) Create a robots.txt file and a sitemap

16) Use Pingdom, Firebug and Dust-Me Selectors to find and fix errors.