Several factors,
Heat! - the only way for solder to want to stick is heat. You would want at least 50+ watts of heating power, Anything less just simply doesn't have enough power and cannot provide enough heat to reach the minimum temperature, you just end up warming it up enough to become very difficult to handle (melts through your table) but not enough to melt solder.
Clean surfaces - ensure the brass pill (it is brass right?) is free of oxide, grease, oil, fingerprints. Give it a scrub with some cotton wool or scotchbrite pads if its really tarnished and covered in muck. it doesn't have to be spotless, but shouldn't be really dirty either
Pre-Tin the parts - before you put it together, leave everything off the pill, and try to get some solder to stick to the rim first, being careful not to allow it to flow onto the fitting surface. Heat up the area you will be soldering, and just try to get the solder to stick first. Do the same for the rim of the PCB. Once you get this done, its much easier to simply re-flow the solder once the parts are together. Again, don't let the solder go where its not meant to.
I would leave the soldering iron to heat up for 5+ minutes to allow it to really get to temperature, and try to pre-heat the brass part first, maybe 1-5 seconds before try ing to apply heat to the PCB, depending on the power of your soldering iron. When you do the PCB, you don't want to be heating up the tracks too much as you may actually melt the tracks off the fiberglass! (thats if you don't kill the components first)
Ensure the tip of the iron is well cleaned and tinned. A clean tip is defined as one that is shiny and takes up solder readily, if the tip itself doesn't want to take the solder, its got too much oxide and needs a wipe/clean/gentle scrubbing. This all depends on the material of the iron too, Plated iron tips must not be filed/scraped through the coating (need a new tip), while copper tips can be filed/scraped a little.
So once you have pre-tinned both sides with a little bit of solder that is Properly flowed (so it looks like its got flat edges, not balls of solder) you then re-fit the parts and simply re-flow the 2 areas of solder together at the same time. Some extra flux here won't hurt, applied before you heat it up.