The Weider Priority Principle is a very old lifting principle that Joe Weider came up with; it's a very common-sense type of law but a lot of people in the gym completely pass it up. Basically it states that you train your weakest or lagging part(s) of your body first, in the beginning of the workout when you're the most focused, and have plenty of energy and strength. For example if your abs/core are weak or lagging behind from the rest of your body then you should train them first, rather then wait till the end of the workout like most do when they're already tired and wont be able to give 100% to training them. I for one hate training abs so whenever I tell myself I have to train abs for the last 10-15 min of a workout a lot of the time they don't get done or they're half-assed. So I make myself do them in the beginning, or I'll do abs on their own day even if only for 20-30 minutes.
The Weider Priority Principle doesn't just have to deal with just lifting though, anything that you are struggling with or are weak in should be moved to the front of the list, it's only common sense. So identify your weak points and begin hitting them first in your workouts and over time you'll find that what once was your weak point is now one of your stronger points!