In the past people reported being able to use the BJ's trials multiple times. I just want to report, that while I have been signing up for free trials every 3 months (Switching off my name and my wifes) when I went in to sign up yesterday I was told that I had one too recently and the lady showed me where it says on the coupon 'for new customers only and one trial per Household'. She did let my wife sign up (even though she had had a previous trial, it was a bit earlier than mine had been).
Basically, just a heads up that it won't always work if you have had a trial recently.