Don't assume you will get depreciation on everything. I claimed the GST on a watch and upon my return a week later I asked the Customs officer if I could claim depreciation. She said no as items like watches, clocks, art etc don't depreciate in the course of a week. I think it's a good point - the majority of depreciation occurs when the good is first purchased and therefore is classed as second hand, and this happens before the time of export. She didn't charge me the AMEX surcharge so it was still worthwhile doing it for the points.
I believe the depreciation percentages quoted by Markis10 are consistent with the guidelines for unaccompanied personal effect importations, which also prescribe the types of goods which cannot be depreciated. I wasn't aware they also apply these percentages to accompanied effects. I hope they do apply this on a consistent basis and it's not an ad hoc interpretation as you can't HUACA your Customs officer.