There are many reasons why women might turn down an offer. Whatever her motive or reasoning, taking "no" for an answer will increase your chances of getting a yes in the future. Or, at least it will save you the bad word-of-mouth that all her hot, single friends will hear. Guys that try and persuade a girl to change her mind are not only annoying and never succeed, but completely close the door on future ass-tapping. Warning: women don't often say 'no' directly. Here are a few variations:

Don't bother
I don't think that's a great idea
Um, that's ok
I can manage OR I'll be fine
Eh (with a shoulder shrug or wince)
I don't feel like it / up to it
It's that time of the month OR I have a headache
Maybe another night
I have a lot to do tomorrow OR I have to get up early
Get away from me you dirty scumbag

For example, if you say "let me walk you to your car" and I say, "oh, that's ok, I don't want to trouble you" that means. . . "I'm not digging you and I'd really rather avoid the obligatory goodnight kiss that occurs in the car zone.' Why can't women just say what they mean? Because we're wired to make your life more difficult.