Samantha,

That is a great question and as a solo entrepreneur myself, I've run into this issue as well. When I first started out, I was keeping track on post-it notes. Crazy right? But I was so focused on the product and making the sale that I was much less concerned than I should have been about keeping track.

I eventually created a spread sheet. I still use a version of the spreadsheet, but I try to capture all my sales electronically as they happen. This could be swiping their card using my Square dongle which generates the invoice or just printing out a paper invoice and keeping a copy until I can manually enter it into the system.

I'm working toward keeping track in a cloud based accounting system, but I haven't quite made it there yet. I'm hopeful to get that done in the coming months.