What are some good methods to populate moderate numbers of pcbs (20-100), without the aid of a pick and place machine?
Avoid through hole parts. Don't put components on both sides of the PCB.
Apply solder paste with a stencil, arrange components, bake/reflow.
(update) Here's a great video series from a guy who built 100 little dev boards in 10 days. http://fletchtronics.net/blog/100-bumble-bs-10-days
If your board is reasonably small, use the soda can method to make a stencil. It's always good to panelize your boards but once the panelized board is over a certain size, i.e. bigger than a soda can, then this doesn't work anymore. Of course, you could always buy a similar thickness sheet of aluminum from McMaster-Carr, then you're only limited by the sheet of paper your laser printer can handle.
For populating boards, save your wrists and your sanity and invest in a cheap manual pick and place. I found that suction pens are ok but nowhere near as handy as a full fledged x-y-z (plus rotation) stage with foot switch for vacuum. There are a few options out there but I found this one to be the best bang for your buck by far. Here's my setup... note the footswitch should be on the floor :)