If you like this, do that.

They say variety is the spice of life. I don't know who "they" are, but I tend to believe them. Especially when it comes to building a successful fitness routine. Do the same exercises for the same amount of time, sets or repetitions for too long and you'll eventually hit that infamous training plateau. You must challenge your muscles regularly, and implementing variety will do just that. Play with speed, incline and resistance on the cardio machines, and try finding new alternatives for your favorite ol' exercises. Some examples: