You just say no. Just make sure you don't change your mind. I've noticed that as "rude" as girls seem when they refuse to give us their number, they take a lot more offense when they want to give it to us for whatever reason (after saying NO the first time) and us guys reject them. They stay sore for a long LONG time and make us seem like the bad guys. I just tell her that the opportunity has passed and if she is nice, I may ask again, but not to count on it. Some guys may not do this, but I feel like I have to take a stand against the whole "I changed my mind now, I'll honor you by giving you my number" thing.
Just be honest, polite and straight to the point. Lying or making false excuses would be poor and pathetic form . also imagine if you were in the opposite situation and how you would prefer to be let down?