My first recommendation is to get a prong collar in addition to your flat collar. The prong is for amplifying communication, not punishment, but they have been known to pop loose. Hence, you will need a leash with dual clips, one for the prong and one for the flat collar backup. There is an Amish flavor that is excellent.
You can buy those at Leerburg.com. There you will find EXCELLENT training videos, many by Michael Ellis, whom I recommend highly. Please take in his video on his philosophy of dog training.
I second Leerburg.com- one of the best sites for learning how to train aggressive dogs and equipment appropriate to the situation.