One of the biggest plateau busters I’ve found in my lifting cycle is using “manuals” or “partner” lifts. Manuals, or at least that’s what they were called in our high school football workouts, entailed either some or all of the workout being resisted by another person (lifting partner).
For example for preacher curls, a manual lift would be when the lifter’s partner would aid the lifter in the concentric portion of the lift and would pull down as the lifter would resist against the added eccentric portion of the lift. This lift is brutal and care needs to be taken by the partner to not pull down too abruptly as to injure the lifter’s bicep.
For rear delts (the muscle in between the shoulder blades) the “lifter” would lay on the bench face down, while the partner would stand straddling their lats. With their arms hanging below the bench, the lifter would press against the partners pressure being applied (with their hands) against the back of the lifter’s elbows. The return to the starting position would be resisted by the lifter, as the partner would push down against the lifters elbows.
Other variations of these types of lifts can be explored in your next lift with a friend. If you come up with other ideas, please list them in the comments section below.