Consider only offering enterprises a yearly pre-pay option if they want invoicing. That reduces your paperwork and lets you put the cash to work. Once you're only dealing with yearly invoices, the collections challenge is simpler.

If the product is SaaS you have the ultimate power to resolve non-payment: stop service.

Just send a polite note to their accounts payable team or whomever is your main contact say "We really love you as a customer, but our invoice is overdue and forcing an automatic stop of service on XXX, XXXX (30 days from today). We really don't wish to loose your firm as a customer. Please contact us as soon as possible so we can avoid a service interruption" Make sure the subject says something like "30 days notice: Automatic Stop of Service on XXXX for non-payment" They should jump to action. If not, remind them again in 2 weeks, then if no action stop the service.

If your service is important to them, you'll be impressed at how quickly things get paid.