This is a simple, yet painful training method popularized by Coach Christian Thibaudeau. You're going to hold an isometric pause at the mid-range point of an exercise on each rep. The length of the pause will vary from rep to rep to accommodate the growing level of fatigue. This method is especially effective for isolation exercises. Three to four sets should do the trick.
Here's what a one set looks like with preacher curl. You'll pause about halfway up:
Obviously, you'll need to use less weight than you normally would for 6-rep sets. (You would've figured that out really fast anyway.)
Thibaudeau likes this method because you can recruit up to 10% more motor units during an isometric action than during a dynamic action. He also likes to watch his clients pass out and choke on their own vomit. Coincidence? We think not.