Lost track of a $3k invoice for three months - learned that lesson fast.
Now I mark two dates on my calendar for every invoice: due date and follow-up date (5 days later). When follow-up day comes, I shoot them a quick email asking if they need anything to process payment.
Make it automatic so you don’t have to think about it. I also set payment terms upfront now instead of hoping clients pay on time. Most people are fine with a clear 15 or 30 day window.
Once you fall behind on follow-ups, it gets awkward to suddenly call about old invoices. Stay on top of it from the start.
I send out three copies of each invoice. Keep one for my files, client gets one, and the third goes in a folder sorted by due date. Every Friday I check what’s due in two weeks and shoot a quick reminder email. Payment comes in? Toss that third copy. Anything left in the folder gets a phone call the day after it’s late. No fancy software needed.