The secret to the process is using a controlled heat source and spreading it out. In a word keep: it moving, keep it rotating. (OK, that was more than "a" word)
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Step 1: Heating the Pipe
PVC is a thermoplastic - which means that it becomes soft when heat is applied. You can use this little tidbit of knowledge to heat and re-form the pipe to a new shape without compromising its physical properties ... if you are patient.
The best tool for heating the pipe is a heat gun - available for around $25 from your local hardware big box or online (search: 1500 watt dual temperature heat gun). You can also take your piece of pipe into the kitchen and use your stove top - just make sure your wife isn't home. What doesn't work so well is a propane (or MAPP) torch as the heat is too concentrated and charring the PVC is highly likely. Save the torch for making your PVC diggerydoo ... but that's another instructible.
The key to the technique is constant movement - move the PVC pipe or the heat source back and forth over a 12" or so area and keep the pipe rotating all the while. After 3 or 4 minutes, the piece of pipe will start to sag. Keep it moving and rotating and begin bending it while rotating it - all the while keeping the heat on.
Step 2: Finishing the Bend
Remember: your heat gun is also your friend when running PVC pipe (or conduit) and you have to make that ~12 degree angle in order to get it into that next fitting - even if the pipe is already in the trench. Just remember to keep the heat source moving - but please don't try drying your hair with it..