Even if you have "the best" workout plan in the world it won't be long before you notice that you're not getting the same results you did when you first started. This is often the result of two main culprits, 1) your body has already made the necessary changes (adaptations) to the stressor (exercise routine) and doesn't need to do anything more (you've plateaued), or 2) you may be overtraining, meaning that your body needs to rest.
Fortunately, the fix for these two problems is not too hard to overcome. Follow these simple steps to fire things back up and move to the next level.
1). Rest. If you've been going fast and furious for a few months take a couple of days off. To keep you from losing your fitness habit, use your normal workout time for more calming activities such as stretching, yoga, or a massage treatment.
2). Think outside the box. Cross training can add depth and purpose to your fitness routine as you use your new-found strength and abilities. Find an interesting trail to hike, go for a bike ride, try a new activity like rollerblading or indoor rock climbing. The goal is to do something new and outside of your norm.
3). Change your routine. If you've been working the same exercises, in the same order, and for the same number of set and reps...your body is bored. Generally, the average non-athlete should change routines every 4-6 weeks, if not more often. This doesn't necessarily mean that you need to get rid of everything, changing the order or simply adding a new move could be enough. Additionally, you can work on slowly increasing the amount of weight you move or the number of reps/sets performed (volume). Note: don't increase volume and weight together; increase the weight OR increase the volume to help you stay injury free.
4). Look at your calories in versus calories out. If your body is no longer leaning out or building muscle progressively, your diet could be the culprit. To lose body fat, you need to create a calorie deficit ideally through exercise AND eating. Generally, that equates to eating slightly less, exercising slightly more. If building muscle is the goal, you need to eat an adequate combination of complex carbs and lean protein; "FEED the muscle" as they say.
By focusing on these tips you can give your body new stimuli, thus encouraging your body to begin creating new adaptations to push you past your plateau and on to a new level of fitness.
Arms a little too jiggly? Summer tops feeling a bit snug by the end of the season? Beginning to bust out of your summer bathing suit? 
