I wouldn't put in the "I can't stay too long." Think about it. Makes no sense to say it in this situation. That's good in a social scene, with friends around, or if you're at the park or soemthing and need to watch your little sister. Just go with the original line and leave out the time constraint. It's understood that you're shopping.
Watch this video by Richard McIlwaine.
http://www.puatraining.com/puablog/2008 ... ful-women/
Open and then after you get your question answered, just continue the conversation. You're not definitely (you might tho) going to get it the first time, but go with the flow and work on it. I'm working on this now.