I've no idea whether I'm doing it right - but this works with my 75 watt iron. I agree, the rail and the bit both need to be as clean as possible and then I 'tin' the rails and the wire used for the jumper.
I apply the iron to the rail pressing hard for around 3 seconds, and then start applying the solder between the iron and the rail. I find the melting solder seems to increase the heat flow to the rail and the solder then starts flowing along the rail as the heat spreads. In a few seconds (usually less than 5) there's sufficient solder on the rail so I can remove the iron. Not had a problem with melting the sleepers.
Occasionally the solder doesn't flow, usually because I've not cleaned the rail sufficiently well. So I start all over again, giving the rail an extra bit of a clean. I use a triangle section needle file with a squared-off end. That way I can really get the oxidisation layer off the brass. After tinning the ends of the wire I hold the wire against the tinned rail (with pointed nosed pliers) and apply the heat again with a bit more new solder as above to assist with the heat transfer. Usually have to hold the wire in place for a second or two while the solder cools (usually assisted with some gentle blowing).
Seems to work for me. Just the occasional dry-joint but most of them have been out there now for around four years and are going strong.
No idea if I'm breaking any of the golden rules of soldering - but it seems to work for me!
Rik