My experience in HK regarding walkups (other methods such as Honey Girls would be different) was to visit the buildings and ring bells. Ring enough and it's very likely you will find what you're looking for. I never felt like I was being scammed or anything. The buildings ARE nasty on the outside, the hallways, and the elevators, but the rooms are clean (though bathroom fixtures have seen better days).
I'll also throw out the suggestion to check out CC
http://go141.com/en/A22662-TST-1 ... ml?ref=Search(stype,region) if you can. Easy booking, very friendly. Just expect a kiss with tongue within about five seconds of walking in (that was a first for me with a WG)!