Intro: Using Fun-Tak to Assemble PCBs
Holding through hole components in place while you flip the board upside down is a perennial challenge. Most people suggest bending the component leads, but this is not an ideal solution. First, it only works for components with long leads; it doesn't work with IC sockets, headers, etc. Second, components with bent leads can still come partially out, leaving you with a resistor up in the air when you are done.
I offer a solution to these problems - FunTak. It is a mounting putty commonly used to mount posters on a wall. It is meant to be removable. To use, simply make small amounts into balls and attach the component to the PCB with it. Keep it away from the leads - it doesn't handle heat well. The photos show this being used with a 4-pin male header. Your component will stay securely in place until it is soldered.
FunTak tends to get sticky when warm, so let your components cool a bit before removing. Sometimes, some of it will remain stuck to your board or component. Fortunately, it sticks better to itself than anything else, so just ball up the wad you removed and use it to "blot up" any remaining putty.
There are many other uses for FunTak. One I found particularly nice was to hold magnet wire on the spool (shown in the last photo). Tape tends to get lost and/or loose it stickiness. FunTak is superior on both counts. I am sure you can find even more uses for this substance. Be sure to buy the Loctite brand. I found other brands to not work very well.