I constantly forget to carry all my membership, club, discount, and scan-this-when-you-buy-something-and-get-points-that-you-don't-really-know-what-to-do-with cards. But I do remember to carry my iPhone. Solution? Use the free CardKing app to scan my forgotten cards and pull up the barcodes when they're needed.
Sure, it might be a bit awkward to hand my iPhone to sales people so that they can scan the barcode and maybe the reflection on the screen will prevent things from scanning properly—in which case I'm sure the barcode can be hand-entered—but darn it, at least I'll be able to claim that I didn't forget my random cards. [CardKing]