Hi Joanna,
I rely a lot on labels, so the place looks a lot like a mouse cemetery
I try to keep a record of plants I buy and often (but not always) record where in the garden I've planted them.
Being very last century I still use exercise books to do a lot of my recording and keep an annual diary for gardening to record when new flowers first open, what seeds I've sown, that sort of thing.
I've often thought of using index cards but I suppose I should invest in some sort of recording system on computer. I'm sure there's a spread sheet or similar program out there somewhere!
I believe keeping records is important but even in a small garden it's easy to lose track of things and sometimes it's interesting to find a record of a plant in the diary and think "now where did I plant that?" and hunt around to find it! Sometimes I just find the label
Whenever we renovate an area of the garden we uncover labels that make me realise how many things we've planted and how many haven't survived. That could be depressing but most gardeners get a healthy dose of amnesia when it comes to plant losses!
cheers
fermi