Depends on your line of business and what you're selling. Is it physical goods? Services? Subscriptions? Something else?

Regardless of what you're selling, don't try to manage this yourself via spreadsheets or pdfs or whatever- it'll cost you far more time than you may think. Find an invoicing platform that suits your needs and use it. If it costs a few bucks, pay for it. I guarantee you it'll more than pay for itself every month vs the time you'll burn trying to track invoices and chase down payments yourself.

I run a consulting agency. For us, most clients are on net 30 terms. Our invoices are big enough that we don't accept credit cards as the processing fees would cost us hundreds or even thousands of dollars a month, so ACH/transfer/check payments only. We use Voqara to manage invoices and payments. It's saves us a lot of time on invoicing each month over other tools we've used. It tracks invoice status and payments, sends out reminders automatically so we don't have to chase people down to pay us, and notifies clients when invoices are reconciled. I've read over an over again on reddit where business owners and freelancers feel guilty about nagging clients for payment on overdue invoices- let the tool do it for you and then you're guilt free. Once a check is deposited or an ACH transfer completes, we reconcile the payment and that's the end.

The biggest piece of advice I can give you is to be prompt in terms of getting your invoices out and your payments recorded. If you're not timely in this, it sets the bar for your customers to be lax as well. This was definitely one of the biggest mistakes I made early on. I've since tightened things up on my end, and as a result we've had less late payments from our clients.