I hate saying no to people. I do it when I am protecting myself or people I love. A lot of it depends on what is being asked of you.
If someone asks you to help them with babysitting, a little gas money, borrow your cell phone...that's nothing. I usually do that as long as I have what they need to give. I don't do loans. Just gifts.
This is from a chick perspective... If someone is begging and pleading for you to have sex with them like a used car salesman trying to sell a car (99% of the men out there on a date persay), then you are giving up a lot more, and therefore, should never feel compelled to say yes no matter how hard (no pun intended) the guy will make you feel compelled to do it.
Other examples of where you are protecting yourself is saying no to illegal and legal drug use, no to drinking when you got young ones under your care, no to loaning out the money for which you plan to pay an important bill with, etc. Basically, the no-brainers.
I also find it easier to say no when people abuse my kindness, like that neighbor that comes by 6-10 times a day asking to borrow the phone, the computer, the internet, some cash, watch their kid for minute, cook their kid supper, give their kid a bath, drive them somewhere, let them borrow your car, and seem incapable of solving any single problem they have without your help. No matter how bad I may feel for their kid, if I'm taking care of their kid like that, then I should get to keep the kid.