This question boggles even the mind of pro athletes and bodybuilders at the gym. Some of them resort to cheating i.e. taking steroids and prescription drugs to enhance their body's lean muscles. I don't recommend the latter simply because it isn't healthy. You're basically cheating your own body to look and feel good outside. The problem here is after you discontinued using steroids, your body will return to its previous state. All those gorgeous muscles will be gone in an instant. Plus, there are proven side effects other people can attest on using these drugs i.e. depression, anxiety, hair loss, gynecomastia (man boobs), sexual dysfunction and other things.

The thing here is that after months or years of training your body has adapted to the level of stress (via weight training) on your routines. Our body has some sort of memory which means it can remember and adapts to each exercise you do. Try to imagine this as our body's way of perfection and healing itself. Your body grew bigger and stronger after your first 6 months of training because your body wasn't accustom to the task (exercise) you did. Most probably, the set of weights you struggled 6 months ago is easier to carry now.

In order to reverse your body's plateau you need to find a way how to "confuse" your body. By doing so your body will not recognize the stimuli you employ and therefore it will respond positively. There are no complex rules and methods here, just plain common sense. You need to change everything that you do at the gym, from number sets and repetitions on each exercise to exercise routines, equipments that you use or the environment i.e. changing gym location or changing the bench inclination in doing incline bench press.

Plateaus can be observed in muscle gains or weight loss. The key is to have "variety" of methods. Enjoy doing what you do and surely you'll achieve success in your workouts.