How can i be mysterious?

Ok so im not ugly and i have guys that like me, but most people do, but around guys especially REALLY cute ones im really nervous and just a mess! i want to be more mysterious and not so obvious that i like them, yah no keep them guessing! when i first talk to guys i can act like myself (sarcastic,funny,werid:)) but as soon as they start flirting or i find out they like me, i get nervous and become really......UN-COOL! please help! something i tell myself or tips to keep them guessing while being able to be myself instead of a blabbering-idiot!