How do I avoid a pinch puncture? Newly mended inner tube went down within 15 minutes of pumping up and without any riding or even sitting on bike. Handpump only so no idea of pressure. What did I do wrong/ how can I avoid it happening again?
Most likely:


You didn't repair the original puncture properly, and air was still leaking from that site.
Whatever caused the puncture in the first place is still in the tire, and re-punctured the tube. 
There was more than one puncture, possibly a small one that you didn't notice when finding the major one.


Possibly you also nicked the tube with a tire lever when getting the tire off. I think this might be what you are alluding to in your title.

By the way a 'pinch' puncture or flat usually refers to a puncture caused by the wheel hitting a bump that deforms the tire so much the tube is pinched between tire and rim. Frequent pinch flats indicate tire pressure is too low for the surface being ridden on.