I use a combination of tools. First off and most thoroughly, I use a website called MG Experience (MGExp.com) in which I have my own home page and Journal.
The Journal is done by phase (such as engine, suspension, interior, body, etc). Each phase allows me to put in multiple photographs and to use text to describe exactly what I did.
Additionally, for major things (such as a high performance engine rebuild), I keep the receipts in the master folder in my file cabinet.
I also keep various mini binders with schematics for such categories as altered electrical systems, engine performance changes, altered engine compartment, etc. either in the garage or in the car itself. This is particularly important and helpful since my classic is a bit of a daily driver and if I have road problems on one of the altered circuits I’ve done.
Despite what some others have said, I do keep tallies by general work area (such as engine internals, engine externals, engine compartment, interior, body, suspension, etc.) on an Excel spreadsheet. That way I can see the total investment by exact item and category for everything I’ve done as well as crossfoot totals in each area by vendor for the life of the automobile.
Yes, the results of those totals can be shocking but it helps me appreciate the totality of what I’ve done and brings back some pleasant memories of doing the work itself.