First, train your lagging body part at the beginning of your workout. This way, you can dedicate all of your focus and strength to the body part in question before you’ve trained any other parts. This will eliminate the possibility of giving a sub-par effort due to exhaustion from other exercises.
The priority principle doesn’t end there, though. Improvement will come from working the lagging muscle group early AND often. There are a number of ways to add the “often” element. One way is to essentially make every set a superset, returning to the lagging muscle group after one set of whatever other exercise you perform.
Another way is to intersperse a set or two for the lagging part in between each of your other exercises. This way, you will be drilling the lagging muscle more than your other muscles, but still allowing time for rest.
The idea is to get that muscle moving as much as possible, in my case is the stomach/abs, so I try to hit those at least three times a week either for 15 mins at the end of my workout or super setting in btw exercises.