"Insanity is doing the same thing over and over again
and expecting different results." -Albert Einstein
I can't tell you how annoyed I get when hear my friends complain about how great their workout used to be, but it stopped working for them, because they are getting older. Or, how they have been a swimmer all their life, and it's the best workout, so they don't do anything else, but they've put on 10 pounds.
Honestly, the reason why workouts stop making such dramatic impact is muscle memory. Your body will always automatically modify itself to find the easiest way for it to perform a given movement. It's genetic! It's part of what makes human's so adaptive and efficient as a species! But, it also works against you when you work out. This is why it's so important to vary your workout, and change what you are doing constantly. This doesn't mean you have to suddenly take up thai boxing or rock climbing to keep your body on it's toes (though, those are both great activities which I encourage you to try if you have not).
By adding weight to a movement, you make it harder. By adding variation to a movement, you make it EVEN HARDER. Adding weight to a workout it the BEST way to maximize it's impact, even if it's just 5 pounds. It can make a beginner movement advanced, and it can help you add variation of movements to your routine!
If you do yoga, VARY YOUR PRACTICE! take twists where you haven't, try flipping the dog, work in a new balancing pose. It's the same concept for any workout you do.
My Problem Area: Lower Ab's
The place where I have found the most difficult to get that after workout burn no matter how hard I push myself, is in my ab workout (and believe me when I say, I have no 6 pack abs). I do it all! I've tried everything! I do every kid of ab workout under the sun, 7 days a week, and I still found myself developing that lower belly pooch which we all complain about. I admit that I want to complain that it's because I'm getting older but I refuse to accept that. I finally found the best and most consistent results when I strap on a extra 5 pound of ankle weights, incorporate a medicine ball twist here and there in my routine, and mix up my routines regularly. I feel the burn, I see results, and so will you!
Bellow are some links with great movements that you should try incorporating into your workouts, if it's too easy for you and you're not feeling the burn- AD SOME WEIGHT or VARY IT or BOTH!