I have Google sheets for both my catalogue and my wishlist.
For my catalogue, I have tabs for Originals and main sets, boys, babies, sea ponies, rainbow ponies, flutters and wingers, playsets and non-pony, and lastly families.
That way, I can access the sheets on both my laptop and my phone, so if I've come across a pony at a charity shop or car boot, I can check my lists whilst on the go.
I have journals/ scrapbooks for my customs, and once I've done a fair number of them, I might even treat myself to getting an album done of my work off photobox!