Select a filler metal with a melting point between the melting points of the two metals. When welding, place the filler metal adjacent to the metal with the lower melting point. This way, it can act as a buffer and protect the metal from the effects of excessive heat.
Preheat the metal with the higher thermal conductivity. This will avoid the possibility of that metal drawing heat away from the other base metal and will make for the more even conduction of heat. Avoid softening the metal or altering its mechanical properties; consult a welding professional for suggestions about the specific amount to preheat a particular piece of metal.
Select a filler metal with a thermal expansion coefficient between those of the two base metals. Such a filler metal will minimize cracking caused by metal shrinkage when heat is removed.
Post-heat, if necessary, to remove the cracks that result from differential expansion coefficients. Avoid overheating one of the metals in an effort to fully anneal the other. If you choose to post-heat, consult an expert as to specific post-heating temperatures.