Keep an umbrella around on a rainy day. Teasinly ask your 'friend' if they want to borrow it. but make them work for it. run around in the rain, keep it from them, anything to get the flirting started.

i did this before and when he stole the umbrella from me i told if he didnt give it back i would kiss him. Instead of me kissing him he kissed me the moment i said that. We are still together today.(3 years later if your wondering)