Keep in mind that Amazon still doesn't collect tax for out-of-state third-party sellers (unless the order is "Fulfilled by Amazon"). From your Amazon account page, you can download your annual purchase history in spreadsheet-compatible format, with the taxes broken out into their own column, and use that to figure what purchases still need to be accounted for for calculating use tax. That was more necessary back when they didn't collect any California sales tax at all, but still comes up if you're buying from third-party sellers.
Note that if you haven't kept receipts to calculate your use tax, you're supposed to estimate it as (for 2013 in California anyway) 0.033% of your adjusted gross income. Depending on your spending, that works out to being as if, very roughly, 2% of your spending were out-of-state.