How can I teach my less-skilled friends the value of scouting and aggression?

Solution 1:

Get them into watching replays by Husky or HDStarcaft or Day9 watching the pro level play gives you so much insight! Not to mention that they somehow manage to make it exciting to watch a replay.

Then after you watch a replay, do a custom game and have them try some of the tactics from the video, and you and your friends can both improve your skill at the same time.

Solution 2:

In my experience it's not hard just to see the value in scouting and aggression. You don't need to play online long before you get frustrated with rushes and incoming attacks that harass you or do real damage to your army or base. It's easy to flip that around and say, hmm, maybe I can do the same sort of thing if I get better at launching an attack.

And everyone has had the experience of running their army into an enemy army that perfectly counters it, where your troops just seem to melt without doing any damage at all. You flip that around by saying, hmmm if I knew he had Vikings, I wouldn't have gone heavy Mutalisks... and so on.

But for teaching how to do this, I recommend the resources jblaske posted, a well as SC2 Noob School by Trebis at Team Liquid. Check out his forum thread with videos and commentary, and the SC2NoobSchool YouTube channel.