Its the mains that produce the most spin in your racquet, so stringing the mains lower will produce more spin.
But on the flip side, you also have to loosen your crosses too. Correct me if i'm wrong, but when the crosses are tighter than the mains, that will raise the overall reference tension... sort of undo-ing your efforts in loosening your mains.
If you're stringing at 53/52, then bring both down at least two lbs, if not more, together, in order to increase spin.
And, perhaps string a smaller gauge too. The thinner the string, the more spin.