Use heavy negatives on the dip machine for triceps. I absolutely love doing these. It's an effective, safe way to overload your triceps with heavy loads.
Have someone help you do the concentric portion of the lift. Then do the eccentric or negative portion by yourself. Take a few seconds to lower the weight.
Anytime you plateau in training, it's rarely because your program isn't complex enough. Usually it's because you've strayed from the basics. Here's an overview of the basics of training.
If there's one constant in strength training, it's variety. Those who vary their programs will often make consistent progress. What's common in most programs, however, is a lack of variety!