1. Put your mind in the muscle – When you perform your weight training exercises, really zero in your concentration on what you're doing. Cancel out all outside distractions as much as you can. If it works for you, plug in the ear buds and turn up the music. When doing your exercise's visualise and think about the muscles you're working. Feel the muscle fibres contracting and really 'squeeze' the muscles tighter as they contract. The more you use this technique the more muscle fibres you will recruit. Over time you can actually increase the amount of muscle fibres you recruit when you lift. Performing reps like this results in more complete muscle activation and greater overall development. Flexing your muscles between the sets is a good way to get a 'feel' for the muscle fibres you are working on. You can feel them more as they get more pumped through the workout.
2. Use Drop Sets – Drop Sets are when you perform reps of an exercise until you reach failure or get close to failure, then without stopping the exercise, drop the weights down to a lower weight and immediately continue on with more reps until you reach the point of failure. Bodybuilders love Drop Sets because they are helpful for building muscle size and shape, as well as burning more calories (if you are dieting or cutting up). Drop Sets are a great way to ramp up intensity, to the point where you will feel like the muscles are burning and on fire! They're not as effective at building strength and power though, the next tip is best for that...
3. Use Eccentric Overload – You need a partner for this one. The eccentric part of a weight training exercise is when the muscles are lengthening. It's also the part of the exercise that causes the greatest potential for muscle growth. So for biceps curls for example, the eccentric part of the exercise is when you lower the weight down. Your eccentric strength is up to 40% higher than your concentric strength. The concentric part is when your muscles contract, so for biceps curls this would be when you curl the weight back up. To use Eccentric Overload, load the bar up with a weight greater than your one rep max. Have a spotting partner help you raise the bar up through the concentric part of the movement, then slowly lower the weight over about 3 to 5 seconds unassisted. Still keep your spotter focused on you there though. Repeat. Perform your reps like this. This type of training will cause massive overload on your muscles, and has great potential for strength and size development. Restrict eccentric training to about 1 week per month.
I hope these tips may help you boost your training intensity. Let me know if you have any questions.