Cleanliness is paramount of course; wash the surface with detergent, solvent (acetone, say) and etch or activate with whatever is appropriate (a mild acid, or complexing agent, will remove oxidation; I don't know if Cu/Ni needs an activator but I can imagine there may be things for that too).
AFAIK, nickel should go on thin, because it does indeed tend to curl off the surface. Too much internal stress. I don't know offhand if there's a way to control that, or if there's a bath or alloy mix that behaves good enough for thick layers.
If nothing else, you could plate a little, anneal, clean and repeat, but that would be awful...
Are you using a commercial plating bath, or winging it from chemicals? If the latter, good purity is needed, and probably some handy additives (a leveler like thiourea for example, at least for copper, but I imagine it works for nickel as well).
Tim