Sending Invoices in a Timely Manner

For one of my businesses I’m self-employed and need to send out invoices to get paid.  Recently I’ve begun to try to send the invoice right after I complete the work.  If done quickly, it just takes about 5 to 10 minutes.  I’m very, very behind on sending my invoices.  What would you suggest on how to get current and stay current?

The delay has become a problem on many, many levels.  I think now that I’ve begun doing the invoices right after the job, or at least the same day, that is helping but I need to catch up for months and months and trying to figure out some of the billing issues will be time consuming.  Thank you.