Once you have earned the right to increase the intensity of your workouts.  There are several ways to approach increasing intensity. You can use heavier loads, more aggressive contractions, and/or adding intensity by heavily overloading particular points of the range or phase of contraction.  Here is how I would recommend you progress.

Step 1

Increase the load, but maintain a controlled steady tempo.  The easiest progression for your nervous system is to keep your speed of movement controlled but simply increase the load.  None of the skill part of the exercise changes, it is simply just heavier, which will allow you to produce more tension against the resistance.  The weights will not be extremely heavy yet, but may feel very heavy because you are going slow and eliminating momentum.

Step 2

Learn to increase intensity within a contraction.  Maintain controlled initiation. This will ensure that you are still using the target muscle.  Once you have initiated the concentric, try to accelerate the weight by contracting harder. This is a great way to get your muscles better at pushing through heavy loads and fatigue without losing tension in the target muscle.  It’s important to make sure you intensity does not exceed the weight you are using. Too light of weight and the increased speed will result in too much momentum and loss of tension. The better you get at accelerating the weight, the heavier weights you will need to resist that acceleration, decreasing momentum and providing resistance against which you can apply more and more tension.

Step 3

Contract aggressively from the initiation.  One of the most important parts of execution is initiating with the working muscle, and this is the hardest portion to progress.  If you think about trying to move the weight faster from a dead stop, you’re going to cheat. You really have to focus on just contracting the target muscle harder from the get go.  If you screw up initiation, especially with an aggressive movement, there is no chance you will regain tension in that muscle. Again, the weight and the intensity need to match. You will have to use heavier weights.

Step 4

Use heavily overload portions of the strength curve.  Unfortunately many exercises, especially poorly designed machines already have heavily overloaded portions of the strength curve, which is not effective for novice trainers or conducive for learning better contractions.  By overloading particular portions of the strength curve, you can create some brutal hypertrophy stimulating supersets to extend your time under tension during high intensity workouts.

Don't Rush Yourself

Remember that your goal in training is to provide just enough stimulus to get you the stimulus you need to grow.  Don’t think that rushing into the most advanced training methods is going to get you better or faster results. It’s likely to do the opposite.  If you take your time and progress in this manner, you should be able to maintain your execution. You will also make continued progress by being able to progressively increase the intensity of your sessions.  Don’t lose the skill and discipline you have worked so hard for thus far. Embrace the process, and you will reap the rewards in pounds of muscles.