I've repaired my pipes for years now and actually buying the proper fittings goes a long way but it can be difficult trying to revive some dents especially close to the header area. The fittings is similar to this
http://www.mino.nl/components/com_virtuemart/shop_image/product/Pipe_repair_kit__54fdf53f993ee.png
My method to blowing dents out is:
# Have a tyre gauge that can deflate and stay connected to the tyre valve whilst connected to a compressor - you need to monitor and control/deflate the pressure.
# Attach the fittings and if your using homemade jobbies makes sure lock wiring the fittings to say a hose clamp especially at the stinger end.
# Pump 20psi and perform a leak down test, you shouldn't loose a psi in 5-10 minutes, if it does then you'll need to mindful of the pressure loss rate.
# Need a map gas, propane won't get hot enough unless you live in the desert and can keep the metal hot.
# Pump to 20psi, and work the dent in circular motion, working outside in getting the dent red hot.
# Keep an eye on the pressure gauge - My first attempt l put too much air (80psi) and saw 150 odd psi before l split the seam where l was heating.
# The dent should slowly rise, if it doesn't add 1-2 psi at a time keeping heat and watching it rise slowly - never feed air @10psi at a time (see above)
# Deep dents especially close to the header end, will require heating the middle first and working to the outer (again wait till it slowly rises).
# To get a close to perfect surface, let the pipe cool down whilst keeping the pressure then reapply heat - constant heat around the problem will effectively weaken the metal and thin it out causing a blow through to a split.
# Once finish, let the pipe cool down natural before using scotchbrite to add the shine back on.
All these vids that state pump 40-60 psi is a precursor to splitting your pipe as the pressure will rise faster and if you can't control the the rate that the dent rises, you can split the pipe or worse, burst fittings and the pipe - patience rather than quick fix.
You can use the freezer method as well but you bloody better have a big freezer and incorrectly filling the pipe with too much water will burst the pipe faster than the heat method. Also, you can use dry ice as well as it will make the steel contract quickly and then relax itself back into shape.
DISCLAIMER: If you chose to repair your pipe using heat, please wear appropriate safety gear (l use a welding masking and leather apron) and l won't be liable if you cause injury to yourself, equipment or others.