Your best friend is the encirclement.
What you need to do is take a few corps of tanks (a few north and a few south) and send them deep behind enemy lines (not too deep though). Then they should meet up at a point and all the enemy divisions in the "pocket" should now be out of supply and should fall very easily to your power.
Remember to keep guards back to defend your advancing tanks. If they get cut off, you'll lose all of them.
I'm sure someone else can give a much better explanation.
Also, look at the wiki (The link is in my other pants). It has a pretty good article on blitzkrieging.