I actually thought that the #8 alliance at MSC did an amazing job with three robots scoring. Granted most alliances can't really pull it off.. but, man, #8 was crazy.
As to effective defense. One of the best things you can do is push all of the other alliances tubes into your lanes. This is killer.
Selective tube starvation is also super effective.
http://www.youtube.com/user/FIRSTinM.../1/wlvrf-RtPOE
In this finals match in Ann Arbor the red alliance steals all of blue's triangles. Blue doesn't realize it until its too late and can't make any logos. Awesome stratigy!