First of all, I don't feel guilty if I don't make a purchase. They get enough of my business when I do buy. Secondly, strangely enough, Walmart has the correct greeter technique. The guy at the entrance welcomes you to the store and asks if you need assistance. If you say no, he'll leave you alone.

The last time I was in the big chain hardware store:
"Can I help you?"
"Nope, I'm good."
"Well, it's store policy that we help the customer."
"In that case, can you give me directions to a hardware store where it's store policy to respect the customer's wishes?"
He got the message and left me alone.