Adding more weights to the same ol' workouts doesn't really do much to "switch things up". You're still doing the same workout, working the same muscle fibers, and you'll not really accomplish anything more.
Have you ever read the comments on here where someone says, "I haven't done such-and-such a workout in months, and I did it last night and you wouldn't believe the DOMS I'm feeling today!"? I mean, they may have been doing PLB twice a week for two months, and the DOMS stopped hitting them. Then they put in Leaner Legs or Legs & Glutes one day and BAM! The DOMS hits 'em hard! It's because Leaner Legs and Legs & Glutes hit the muscles differently than PLB does. And THAT is the key!
I used to bodybuild and something I learned was that it's good to mix things up. You find a "rotation" or set of exercises that you want to work with, and you do them for a few weeks. In the beginning, you're muscles are firing and you get DOMS and all that jazz. After a while, the DOMS isn't as pronounced and maybe you just feel like you're not getting enough out of your workouts, anymore. So, you switch to a different set of exercises. Maybe you switch up the order, too. Switching up the order helps shock your muscles even more, because they have gotten used to being trained in a certain order every week. So, once you switch up your workouts, all of a sudden the DOMS hits you again full-force.
Here's an example of how to switch up your workouts:
Weeks 1-4
Monday: Legs - PLB + abs (abs last)
Tuesday: Cardio
Wed: Upper body - PUB
Thurs: Cardio + abs (abs last)
Fri: off
Sat: Legs - PLB + cardio
Sun: Cardio + PUB
Weeks 5-8
Mon: off
Tues: Cardio+abs (abs first)
Wed: Legs - Leaner Legs + 2 exercises from S&H Legs
Thurs: Chest & Tris & Shoulders - Use appropriate segments from PUB + GS: Supersets premix or S&H Chest & Tris (add S&H Shoulders)
Fri: Cardio+abs (abs first)
Sat: Back & Biceps (S&H + GS)+ cardio
Sun: Cardio + Abs + Legs & Glutes (cardio first, abs last)
I love mixing and matching workouts to switch things up. I seem to get good results this way. I especially like breaking chest, tris, and shoulders out from back and biceps so I can focus extra attention on each muscle group.