You won't hurt yourself if you don't do anything extreme. By extreme, I mean, hanging 50 lbs off your dick or clamping for 2 hours a day.
Jelqing is actually beneficial to the health of the penis - the added growth is just a "side effect." Don't use too much tension and start with only 5 minutes per day. If you notice your erection is flagging, then reduce the time or frequency (i.e., every other day, or every three days).
The main thing to remember is to work with a penis that is between 30-70% erect. The only risk with jelqing is when you jelq at higher erection levels.
Well, you could learn how to move dumpster trucks without causing yourself an injury -- but the key is time and conditioning.
Quick hit the nail on the head. Penile exercising is in fact healthy. The hardness and healthy benefits usually outweigh the size benefits (Imagine being able ejaculate without the erection subsiding -- some men experience newfound hardness like this).
However, like regular exercising, PE injuries can happen. I'll be honest with you. Most injuries take place when the exerciser wasn't using caution, or following his body's responses. The best advice I can give is to follow your PI's, and gradually move up in intensity. Here is the PI article that will help you avoid injuries and build a bigger, harder, and healthier penis: http://exercisingthepenis.com/index.php?page=pis