I train weights 4 days a week and cardio/boxing 3 days a week.
The boxing training I do is normally just aerobic conditioning training.
eg: Sparring, specific muscle targeting, jump squats, bob-and-weave, duck-and-weave, speed ball, heavy bag, etc.

The power (force) of a punch comes from many different areas:
The shoulders, primary impact and driving force.
The back, driving force.
The legs, driving force.
The triceps, impact force and speed.
The biceps, retraction force and speed.

All of these areas should be targeted if an increase in punching power wants to be achieved. Each muscle in the body has two different sets of fibers. The fast-twitch(FT) fibers, for explosive movements. The slow-twitch(ST) fibers, for endurance movements. For short amounts of power the FT fibers are targeted. However, if power is going to be need again and again over a extended period of time, the ST fibers are targeted.


Knowing this, there is a technique I use to strengthen the shoulder region, without the heavy bag, using dumbbells.
You will need a dumbbell in each hand with a satisfactory amount of weight on each. To test this, hold each dumbbell out at arm's reach (straight) in front of you. If you can hold the weights for more then 7 seconds the weight is to low. If you can't hold it for more then 4 seconds it is to heavy.

The exercise:
FT Fibers:
1)Stand straight with feet spread shoulder width apart, hands by side.
2)Throw the right hand out straight in front of you, holding the dumbbell (As if you were throwing a punch).
3)Return the arm back to your side.
4)Repeat this with the left arm.
5)Complete 5 sets of 10-20 reps on each arm. 1min rest between sets.
Note: Make sure the punches are explosive, straight and fast BUT always keep the movement smooth and stable. If there is any bone pain or other pain in any muscle but the shoulders, you are training wrong. DO NOT lock the elbows.

ST Fibers:
1)Raise each dumbbell out straight in front of your body. Palms facing down.
2)Hold this position until your shoulders grow sore and you can no longer hold the weights up.
3)Return arms to your side.
4)Complete 5 sets of 6-8 reps. 2mins rest between sets.
Note: The weights should be lifted slowly and smoothly. Lowered the same. Do not lock the elbows at any time. If any pain other then shoulder muscular pain, you are training wrong.

Complete both these exercise 3-4 days a week, with at least 48 hours between training.

This will increase explosive power as well as muscle endurance. Make sure you do both. A strong hard punch is no good if it can not last long.