Not sure how well this will help coz it's a bit risky and depends both on the girl and how smoothly you can pull it off to have the right effect but here it goes. Just randomly say something about some1 pointing out a flaw with them (just make sure it isn't one of her friends) but say it in a slightly over exaggerate but funny way not like the normal "Look at her, she's a fat pie". It's hard for me to explain so I'll give you an example but bare in mind I didn't have trouble talking to this girl beforehand and she knew I didn't mean it in an offensive manner as I've done it before.
We were sitting in English class with a supply teacher and I noticed when she talks she spits quite a bit, on top of that she looks like an old Carol Vorderman and speaks with that sort of classic chav accent, all in all not the best for an English teacher lol.
Neways when I knew the girl next to me wasn't focusing on her work I just said, to nobody in particular "It's like a horrible hybrid between a witch from countdown and a pikie... that spits!" and then continued on exaggerate with things like "my god she just doesn't stop, it's going everywhere like a... like a spit covered drooly.... drooling monster thing." And she burst out laughing and calling me a harsh git while play hitting me before I'd started another exaggeration. (If you can try and keep going while she's laughing, I couldn't coz she'd got me laughing before I could carry on but the next thing I would of said would be something like "I can see it now, we're all gonna die.... every single one of us. If we're lucky she'll stab us before we all drown.")
Basically keep it up and she will start laughing sooner or later but a lot of it is in how you say it rather than what you say. You need to have a sort of serious and sort of disgusted tone but when you emphasize you over emphasize. A quick thinking mind also helps coz if you hesitate at all it won't have the desired effect. Sorry I can't explain it better and sorry I don't have any better ideas but maybe it'll help you come up with something of your own. If not at least I tried
