There is a routine that can greatly enhance one's strength and stamina, and it is absolutely free.
It consists of nothing but push-ups.
It was taught to me by my philosophy teacher, who was ex-SpecOps and ex-NSA, so this guy knows how to quickly build a strong, powerful individual. I believe that it was taught to him by another special forces-type person.
How many push-ups you do is determined by how many (maxing out) you can already do. When I first did it, I was only able to do about 40 at any given time. When I was done, I could do 60 at any given time. I've since done it many times to increase my strength, and now I can do at least 80 push-ups at any given time. I started out doing 250 each day, but now when I do it I do at least 400. The version I'm putting here will start someone out gently, at 100, but I suggest pushing it the first time to see if you can at least do 200.
Here you go:
For 10 straight days, alternate between A and B.
A Day: do 100 push-ups in an hour
B Day: do 100 push-ups over the course of the day
For those 10 days, you should do other exercises, but DO NOT work out your arms in any other way. Just run, do sit-ups, practice martial arts, etc.
After 10 days, rest your arms for 2 days.
After 2 days of rest, drop and try to max out. You should find yourself much stronger and able to do many more push-ups.
You can experiment with what way is best for you, but here's mine:
Do 10 or 12 sets in the hour on A day. Do a set every 5 minutes. (i.e. 25o push-ups: Do 25 every 5 minutes, 10 sets; 400 push-ups: Do 35 every 5 minutes, 12 sets [that actually gives you 420 push-ups], or 40 every 5 minutes, 10 sets)
Do 10 or 12 sets over the day on B day. Do a set every 30 minutes.
There you have it. Try it out. It really is amazing.
I suggest this to all my friends and martial arts students/training partners, and those who have done it have not been disappointed.