There are things to keep in mind:
1) use a good iron (I like the Weller ones)
2) keep your tip tinned and clean (a clean and tinned soldering iron tip is more efficient in heat transfer)
3) use heat sinks (those black and silver paper clips work nice)
4) get some flux (helps the solder flow into the joint)
5) use rosin core solder only (acid core is very bad for electricity)
6) get the track as straight and level as possible
7) wear eye protection
8) all it takes is a little solder
9) clean your soldered joints well after the joint has cooled (remove the flux residue, especially from the rail heads, so that it won't hurt electrical conductivity).
10) let the solder melt into the joint, instead of using the soldering iron to melt the solder directly.
I experimented, and what I found is that I would heat the joint a little, then get the solder out, and when the solder began to flow onto the rails, I'd remove heat, add some flux, and then heat the joint again until the flux had the solder flow into the rail joiners. If the solder was resistant to flow, I would add more flux, and more heat. With some practice, I can sometimes get it done without using heat sinks, but sometimes not. I've got a few curled up ties that will be replaced, which you usually have a spare or two because of cutting the track.