"Have a minute" meetings...not so lean!

My coworker walks up to my desk and says "Do you have a minute, I need to talk to you about something important". He begins to talk about the great deal he got on the Christmas present he bought.
I get interrupted several times during the day by coworkers, people from other departments asking for stuff they need. Being in an open office culture, I have to entertain such requests.

These days many meetings start this way and end up taking more than a few minutes. Even if the actual discussion did not take up much time, you've lost track of what you were doing or thinking. To get back into what you were working on, you need more than a few minutes. Many times you don't remember what you were thinking before you got interrupted.
If the discussion does take a long time, then you've lost time.
In both cases, productivity gets affected.

Most of the times, this meeting may have helped the person who interrupted you. But you did not gain anything from the conversation.
In lean terms, this is waste.

Now you can either choose to be rude and not entertain "have a minute" meetings. Or you can be the nice guy who is ever ready to help everyone anytime. Both these are extreme conditions and have disadvantages.

Couple ways I have tried to prevent interruptions at work are:
1) If you need anything from me and your request can wait, please send me an email.
2) Allocate specific time slots for people to come see me.

I have had some luck with the first approach. Most people in my office have realized that I work on critical stuff that requires me to focus 100%. However, there a few who still cannot stop themselves from coming to my desk the moment they need something.