My trick is to funnel off as much as I can per week without being noticed by the gf into my PayPal account. Then when I find something nice I pay with mostly PP funds and only a bit from the account if possible. Then I show her the item online and how much of a "deal" I got It helps since I'm in the business and can pass it off as a dealer to dealer discount though.
Also, it helps if you have a friend or place you can store your stuff without her seeing it all too. I store a lot of my RC in the garage under stuff or at work and try to swap a few at a time so she doesnt notice. Luckily my gf has no clue the difference between a P3 and KT so I'm all good haha.