I agree it's all in how you handle rejection, but also it's about giving value -- think about how you would want things to go if you were a girl. A guy comes up to you and expresses an interest. You are too busy to stop, or he just doesn't happen to be your cup of tea, or he isn't warmed up and doesn't come across smoothly enough for your taste. You're flattered but you don't want to lead him on or give him false hope. So you say firmly "No, that's okay, thanks anyway"... how would you want him to react? If he gets upset / angry you will also be upset & feel your honesty wasn't appreciated.
Example interaction from last Friday: I saw a cute Asian girl eating alone in the Japanese cafe slightly off campus. Hesitated bit, sat near her, when I finished eating she was still there and I decided to approach. Stood beside her, touched her on the shoulder with back of my hand to pre open, met her eyes "hi there (pause, think I forgot to smile as I was in my head a bit) I saw you were eating all alone, and I decided to keep you company if I can" by this time I was smiling a sexy smile, she said "oh no, that's okay" shaking her head a bit, so I just smiled at her even more warmly, looked into her eyes a bit, placed my open hand on her back and said in a deep slow sexy voice "okay..." and then left the restaurant, I am pretty sure I enhanced her day so this was really just another good interaction even if she wasn't into me. You can do this too.
You can also practice cheeky grins & comebacks to get them to smile a little and maybe even regret later that they blew u out. It's all about getting out of your head and realizing the worst that can happen is they say no, in which case all you have lost is a few minutes of your time, in fact you've also gained because you can troubleshoot, what did I forget to do etc.
cheers, Ray