Be caught looking/light flirting every once in a while- deny it fi she makes an issue of it.
Talk to other chicks on the phone in front of her.
Leave the little “taken” clues that women leave around your house/car there (scrunchies, earrings, etc); if they no woman has done this- buy some and do it yourself.
Don’t answer your phone at suspicious times every once in a while; make up a bad excuse as to why.
Hickies (if you’re into that sort of thing and you’re not exclusive)
“Forget” to clean up before she comes over- leave out two marini glasses and the bottle- one with lipstick marks on it
Take some hair from a brush from a chick with different hair color then hers- leave a few obvious strands where you know she’ll see them.
Leave some empty condom wrappers on top of your bathroom trash; if you haven’t ****ed in a while- just throw out the condoms somewhere else and leave the wrappers
Basicially anything that makes her think that you’re boning other chicks, even if you’re not.