Luminosity and other games along those lines actually will help. So would any hobby or activity which involves mental agility, probably including some video games.

You just have to do a significant amount of it, and the more varied the better.

Navy submarine officers actually practice "mental gymnastics" for this reason, since they attempt to do everything with passive bearings, equivalent to multiple NDB bearings with no DME or other range data.

But like others said if you're a competent instrument pilot you shouldn't have any problem with these tests...airlines aren't trying to select the best "rain-men" they can find, they just want to avoid the far-left end of the bell curve.