Your goal when you walk into the gym is simple: get stronger. If getting stronger isn't your goal, you'll miss out on muscle gains as well as the obvious strength gains. Every muscle building and fat-burning technique is limited if you don't start with a great strength base.
Think of it like this: The person who trains to "build muscle" will do okay for himself and make modest gains for a while. But the person who trains to "get stronger first, then build muscle second" will make better gains and KEEP making them.
Once you build your base of strength then other conditions, like improving your mind-muscle connection, become increasingly important. But with all other factors being equal, the stronger guy is going to be bigger. So increase the weight on the bar, even if it means lowering the amount of reps you do.
The Staple Strength Moves
If you're going to build the most strength and size, you need to put an emphasis on the movements that initiate the greatest anabolic response. These include: the squat, deadlift, lunge, carry, bench press, bent-over row, weighted pull-up, weighted dip, and overhead press.
Without these exercises in the mix, you simply won't make the most progress in the shortest amount of time, or the most sustainable progress over a long period of time. A 300-pound bench press is going to do a heck of a lot more for your strength and physique than a 30-pound bicep curl. If you want gains, you need to use compound movements.
Now, compound movements won't have their fullest effect on your physique unless you put an amount of weight on the bar that has you hovering around the 4 to 8 rep range (or even 1 to 8 rep range if you've got a spotter with you).
For example, deadlifting 225 pounds for 5 reps will only carry your physique so far. If you don't gradually progress to 245 pounds and then 275 pounds and so on, your strength and physique will hit a plateau.
Bottom line: Challenge yourself with heavier loads often.