I used to lift regularly. Few of tips I’ve always employed are:
Get rest. Don’t use the same body part on consecutive days. I used to lift every day (chest, shoulders, triceps, abs on one day, Legs, Back and biceps on the next)
Vary your routine. Do lots of different exercises for each body part. Especially if you use machines, don’t get stuck in the rut of doing the same exercise day after day (switch it up, better physically, and mentally)
Use different techniques. Such as pyramiding the weights (increasing the weight within the rep, until you can only do 2 or 3 reps at that weight, then bring it back down) , doing negative reps (‘cheat’ going up, and then slowly let it down), or other variances.
Go Slow. both in how fast you change weights, and in each movement. ( concentrate on the muscle, not just on jerking the weight up.
Big muscles before small ones. Back before Bi, Chest before Tri. Otherwise the little muscles will get tired, and the bigger ones will not get a workout.
And here’s one very few trainers will tell you. Don’t count. Do your sets until you “feel it”. Depending on the day, and your energy level, and other factors, such as whether you just raised the weight. You may wind up only doing 8 reps, or 15. (If you find you are consistently at the top range, time to change the weight.
Good luck.
Rich