Actually the easiest way is to just say no. Then you don't have to worry about wondering if you should say yes.
One of my guy friends told me that's what spinster women like to do -- they like to say no.
So it's just easier to say no.
For me:
1. Say no to pre-martial sex
2. Say no to kicking kittens, puppies and other cute furry things
3. Say no to guys who do 1 or 2.
But I do say yes to guys asking me out. Ha.