The benefits of training with a heart-rate monitor are well documented. I've only incorporated a heart-rate monitor into my training recently, but I've already learned a lot and can appreciate how heart-rate-based training can benefit my running.
There's just one downside to wearing a heart-rate monitor: the painful chafing that causes awful irritation while I'm running, and leaves scars behind.
After trying lubricants (putting BodyGlide under where the strap and sensor are), or putting soft electrical tape or a band-aid around the edges of the heart-rate strap (thanks to DC Rainmaker for that suggestion), or rotating the sensor around and wearing it on my side, or on a different place on my chest, I was STILL getting chafed. I finally stumbled upon a solution that works for me: leaving my heart-rate strap "as is," and instead, covering my skin with a thin piece of medical tape.
There are lots of different kinds of chafing; I experience it in the center of my chest (and always in the same spot, regardless of where the sensor sits):
To do away with chafing like this, grab a roll of white standard porous medical tape (cheap, available at any drugstore).
Apply the medical tape over any areas where you're being chafed.
I typically use two short strips, and that does the trick. It may wrinkle a bit but that doesn't seem to make a difference.
Then, wear your heart-rate monitor as usual, and enjoy some happy chafe-free running!
See you on the trail!