Holding components without bending leads when soldering upside down?
It isn't a solution to all through hole components, but for devices with more than two pins I've taken to designing my landing pads with hand assembly in mind. For instance, with header, the pins can be slightly offset so the pins fit snuggly in the PCB, like this:



Each through hole has an alternating plus or minus 0.05 mm offset from the center (it's a 1.27 mm pitch header, or 0.05", half the normal 0.1" pitch that's so common). I've produced at least four boards with this augmentation and it works well for holding the header flush while I get it soldered. It even makes routing easier in some cases, like a 45° route from a pad where the adjacent pad is recessed will more easily meet clearance rules.

I haven't used any DIP parts recently, but if I did I would likely use a similar method to make hand assembly less of a chore.

For components with only two leads it's easy enough to tin the corner of one of the through hole pads and hold the component in place with one hand while the other wicks that bit of solder to lightly hold a lead of the component in place. Solder the other lead and then finish up the lightly soldered one.