Go buy some sexy women's lingerie (just tell the employee at the adult novelty shop that you like to wear it yourself, then raise your eyebrows Groucho Marx style a couple of times). Rumple it up a bit and leave it on your bed (along with a large, knobbly adult toy which you purchased at the same novelty shop while once again raising your eyebrows), so your girlfriend can "discover" it. Then say, "Wooops!" and quickly shove the whole lot under the bedsheets.
If that doesn't work, you could actually try wearing the damned nightie yourself as you open the door to greet your gf the next time she comes over. Gripping the aforementioned toy like a bread knife while sporting a wicked smile of pure, furtive lust wouldn't hurt, either.
If that still isn't enough to make the chick hit the road, arrange to have an "escort" come over while you and your gf are making out. Then tell your gf, "I thought I'd have 'Sapphire' here help us liven things up a bit."
If, however, your gf is actually into those things, I don't know what to tell you.