1) Tell her that your going to build a bike for her "to use" (DON'T FORGET THIS PART)
2) Make sure that the bike has a higher bling factor. Yes she will notice this and reward you.
3) Ride along merrily until the relationship falls apart.
4) Exercise legal clause from step one, and then either strip the bling (it's like free upgrades), sell it (bonus beer money), or keep it to cry over.