I try to keep the labels of the plants that I buy.  I try to then log them into a book.  I try to remember where I have planted the said plant.  I try.  The trouble is I wait till I have quite a few labels by which time I could have forgotten where I planted them.  I have started to make a note when planting but I have a tendency at this time of year to move things about and not to make a note of that!  When we first arrived here a bought a great many plants, all of which were new to me, and they were delivered here in one go.  I did not keep a record of what I had ordered but felt that I would keep records and I planted each plant and kept the label.  All went well, plants arrive (all with labels) and were stacked by the greenhouse till the following day when they were to be added to the then new long boarders.  When I came out to plant them I found my 6 month old Doberman sitting on the lawn surrounded by labels!  In fact she had removed every single label from every single pot, all 32 of them.