We all hit a plateau from time to time. Many, many, many of my clients come to me for that very reason - they've been on a plateau for the last 5 years. There are many methods one can use to get off that plateau, but they all boil down to one basic concept: train your body in a new way.
Our bodies will adapt to any work we require it to do for an extended period of time. Whether its what you do for work, carrying a heavy purse, picking up children, or jogging. If you do it often enough for a few weeks, it will become easy, your muscles with adapt to accommodate that mode of "training" and you may see change at first, but by the time it becomes easy for you, your body will have made all the changes it's going to make. You will stop seeing results, with the exception of maybe maintaining what you have achieved - unless, you change it up.
If you've been running 1 mile every other day for 4 -6 weeks, maybe you implement some intervals (work harder for 1 minute, rest for 1 minute) or maybe you change your course so that you have more hills to climb. Either one of those would likely push you past a plateau, but even better than that would be cross training. Cross training is getting an equally challenging workout but using different mode of exercise. Incorporating strength training would be one way to cross train, or doing a spin class, or swimming. There are countless ways to cross train, but you've got to switch it up if you want to continue to see your body changing for the better.
This dilemma is one of the primary reasons that working with a personal trainer is preferable for those who want to reach their goals. Your trainer will continue to modify your workouts as your body adapts so that you don't get stuck on a plateau. As your trainer I would keep your body guessing, getting stronger and leaner. The best part is that it never gets boring! When you show up for your sessions you can trust that it will be challenging and it will be just what your body needs to avoid or bust through any plateau that stands in your way!