One of the problems I encountered in the early stages of my training, and I know a lot of people who have had the same problem, was to maintain motivation while doing the same workouts week in, week out. Not only was this a problem for getting motivated to push myself, my body also grew accustomed as to what to expect, decreasing the effects of my workouts, and eventually lead to plateaus both mentally and physically. Thankfully there are a number of ways to keep your training 'fresh', and to avoid hitting 'the wall'.
The first option, and although this may sound like the laziest option, is in fact rest. Taking a week or so out of training for rest is often over-looked, but not only does it provide the body with the much needed time to recover, and grow, it also helps to re-ignite, for me especially, the drive and hunger to get back training, and break past mental and physical stumbling blocks. Knowing when to rest, and for how long to rest will differ from person to person, and will often depend on other day-to-day activities, such as work, but taking one or two days off training a week, and then an extended break – around 5 days – every month or so, will allow your body to recover fully, and avoid the negative impacts of over-training.
During this rest period, I often have a complete overhaul of my training routines, to keep each workout new, and fresh. The first part of this process involves knowing what your goals are; be it continuing to bulk up, or looking to burn fat, as this will influence your set and rep-ranges. Simple changes such as adding an extra set into an exercise, or decreasing a set but increasing reps every couple of weeks will prevent your body from becoming used to what to expect. Another way of doing this, which I find to work particularly well, is to make a list of exercises, for each muscle group – I have, on average, about 10-12 exercises for each muscle group – and then on the day you are training, pick 4 or 5, and the next time you train that muscle, pick from the other exercises. I like to work off the theory that, if I don't know what exercises I am going to be training with until the day, my body isn't going to know what to expect.
This is only a brief look at a couple of methods to avoid hitting mental and physical plateaus, and there are other ways than these to do so, but it is crucial to avoid letting your training hit a rut, and to keep breaking those mental and physical walls!