You're probably heard that phrase many times. Indeed, it no doubt rolls off your own tongue when a salesperson approaches and you're not ready for a conversation.
While "I'm just looking" sometimes means "go away and leave me alone", it more often means "I'm not ready to buy yet...but I will be later."
Knowing that fact is critical to your success.
Because sometime in the future they will be ready to buy.
The question then becomes: will you be ready to sell?
Unless you've kept in touch with them in the meantime, there's a good chance your customer will go somewhere else.
This is where newsletters (and other keep-in-touch marketing methods) are so valuable.
Because when you keep in contact with someone - not buy saying "ready to buy yet?" but by informing, educating and entertaining - they'll know where to come when the time is right.