Plateau?

One of the best things you can do to avoid this problem is to implement a variety of methods into your workouts. It could be different activities (cycling instead of running), or different training protocols (interval training instead of steady-state cardio). The main thing is that you don’t stick to any one activity or protocol ONLY. When you notice the signs of diminishing returns, adjust your approach accordingly.

For instance, if a 30 minute run just isn’t doing it anymore, try replacing one or two of those runs with a 20 minute high intensity interval workout. Or, if you’re just bored with the same ol' hour of cycling, then signup for a kickboxing class. That's what I do. I do a mixture of Tabata, splits, HIIT, kickboxing, yoga/Pilates, lifting, etc. The key is to keep your body guessing.