A big part of keeping motivated and working out is making sure to avoid a plateau.  A plateau in you workout is when you even off and don't continue to see positive results, whether in weight loss or increase in muscle size and strength. So how do we avoid a plateau.  The main thing here is to make sure you are changing your routines up.  You should also have a plan when it comes to your workout.  What are your goals?  What time frame are you working with in order to reach those goals?  Who is going to keep you motivated?  If you are not sure how to go about how to make up a plan then you need to ask someone in the fitness industry that can help. Training phases called macrocycles that last about 2-7 weeks in general( sometimes a month) are a good approach to devising a plan. Each macrocycle will be made up of microcycles which are one week long.  The structure of any fitness or athletic program should follow this to some degree.  Recovery cycles should be used in between each macrocycle to let the body recover from the adaptations that occurred during the training.  The structure of the phase is based on objectives, the level of fitness, goals, schedule, ect and can vary from client to client. Throughout these phases there can be many variables or stimuli that you can change to make sure your muscles are staying confused and you are reaching your goals.  Some examples would be your recovery times between sets.  Generally if you are looking for muscular endurance you should stick with high repetitions of 12-20 reps with shorter rest periods between sets.  If you are looking to build muscle strength or hypertrophy, you should be focusing on 6-10 reps with slightly longer rest periods. Around your 10th rep, you should be failing or your rep should be very difficult.  If it is not, you need to increase you weight until it is.  KEY POINT:  always proper form before you go up in weight!  If you are looking to develop power and strength, you should have your reps from around 4-6 and failing on your reps. Some other training variables you can use besides changing your weight and rest periods are tempo, time under tension, rest days, recovery strategies, training volume, and nutrition. 

There are several ways to go about changing up your routines and making sure that you are maximizing the specificity of adaptation in order to avoid your exercise plateaus.  Training is an experiment so make sure you have a journal and log to know exactly what you did, how you did it, and what the result was.  Don't be afraid to ask an expert.  Just b/c you have been working out for a long time does not necessarily mean you have all the answers.  Check your EGO and you will surprised on what others may be able to teach you and what has worked for them.

Best in Health,