Yeah I do make each session fairly intensive. I do a something called X-Reps.
I'll explain this but to understand I have to say something else.
Each muscle has two types of fibers in it. Fast-Twitch(FT) and Slow-Twitch(ST) fibers. FT fibers are for explosive sudden movements. They are used for example in punches, kicks and general movement. They tend to get tired quick.
ST fibers are used for endurance movements. eg: Jogging, running, swimming etc.
They do not get tired as quick as FT fibers and use energy slower but for longer periods.

Both of these do contribute to how big, strong, long a muscle will be. Bodybuilders, doing only weights, will not normally train ST fibers, either because more time and patience is required or because they do not need them.

Boxers should train ST fibers though in the case that they may have to go many rounds and still need muscle energy to throw punches.

With all that said, I will explain X-Reps. I will use the simple pushup as the example exercise. However, most exercises can be performed as X-Reps.

Complete a normal pushup as usual. Bending at the elbows, back straight. I'm sure you know how to do a proper pushup.
Continue to do pushups, slowly, until your muscles are that tired that you CANNOT EVEN DO ONE MORE.
Then rest for 30 seconds, no more then this.

Now imagine the pushup is split into three parts.
1)Arms full extended. Chest furthest away from floor.
2)Arms bent 90*.
3)Arms fully bent. Chest almost touching floor.

To complete the X-rep, you would start in position 1, and lower yourself to position 2. Hold this for 3 seconds, then return to position 1.

You do this faster then but STILL as SMOOTH as you would do a normally pushup.
Continue doing this until you CANNOT DO EVEN ONE MORE.

To finish, you hold yourself in position 2 for as long as you can, until you fall to the floor. Laying there, dieing from muscle soreness haha!

THIS IS ONE SET.

I do three sets of these with 2 minutes rest between each set for each exercise I do.
So I know all this might sound complicated and jumbled writing but if you could see it in vision, you'd understand.

BASICALLY all you do is split the exercise you want into 3 parts.
1)Starting Point
2)Midway Point
3)Ending Point.

Then do the full exercise, positions 1-3 until your muscles can't do anymore. Then rest 30 seconds, then do the exercise again from position 1-2 until your muscles can't do anymore. Then rest 30 seconds and hold the exercise in position 2 until you can't hold it no longer. THIS IS ONE SET.

Complete 3 sets of each exercise you do.
This techniques targets your FT and ST fibers together. Works wonders.

Just make SURE that the weight is not to heavy. If bones are clicking/moving improperly or there is any pain anywhere in the body BESIDES the muscle you are suppose to be working, then use lighter weights or correct how you are holding the weight.

Sorry to write so much but yeah it is alot to explain.

About the neck exercise, I find dumbbell shrugs work the trapezius (neck) muscle the best.