Try 63/37 tin/lead instead of 60/40. 63/37 alloy is eutectic, meaning its melting and solidifying temperature is the same (183C). 60/40 has a semisolid state around its melting point (~=190C).
I use kester 66/44 eutetic tin-lead solder on anything I care about.
It's said that light-dark auto helmets make someone a welder nearly overnight; I think that same way about eutetic solder and soldering, it makes the job so much easier that the quality of the work skyrockets.
in racing, they say to drive 10/10ths. not 1's. for the same reason.