This is a pretty basic concept, but I need to be reminded every once in a while.
Switch up your workout routine every few months, or as soon as you notice a plateau in your progress. Doing the same routine week after week will result in less "gains" over time. It's just like the law of diminishing returns - up to a certain point, you will experience less and less progress even if you're putting in the same amount (or possibly more) effort.
I suppose if you're really attached to the same routine, you could continue on the same upward trajectory of progress as you experienced in the beginning if you start working maybe twice as hard.
But since I'm lazy and I'm all about short cuts, I don't want to work twice as hard!
An easier option is to just try something new.
For the past few months, I've cycled through the following workouts:
Abs & core class (pushups, burpees)
Despite doing a different workout almost every day of the week, I recently stopped feeling a "kill-me-now-because-i-can't-walk-up-the-hill-to-go-to-work" type of dread. And this sounds crazy, but not being sore was way worse than being sore, especially after I had invested the time and effort to working out. (And we all know if you're not sore, you're not progressing.)
At first, I told myself that the lack of soreness just means my body is becoming stronger, so yay! Keep doing the same thing! But no, that's not the right way to look at the situation. Instead, what it really means is that I'm wasting my time if I stick with the same workouts. Sure, I may be stronger, but those workouts are no longer beneficial and they are not making me stronger anymore. Hence, the plateau.
I took a big leap of faith and signed up for a week of girly classes that I used to make fun of: barre classes and pilates. I've progressed more in this one week that I have this entire month. It hurts to laugh and I had a hard time bending over to pick up golf balls today. Never again will I criticize girly classes. That is, unless those are the only classes you take. :P