2 steps
Since it's already happened to you changing policies won't change the past, but it can change the future. Here is what I recommend.
1. Yes. Change your policies so you don't go through this again. Cash upfront or on completion of job. I may not be easy but you must do it. Don't give a way out such as, "Sure payment when you pick it up. Oh, you'll have the check in the morning. No problem." Don't give your work until morning then.
2. For existing Accounts receivables go after them now. Email, call, drop in. Don't use the excuse that you are busy. If it gets too far behind, it's more likely to lose a good customer who just lost an invoice or didn't receive it. I had this happen.
I know some of the above responses said mail worked much better than email. I don't find this to be the case. In the 12 months from October 2013 to September 2014 I counted about 180 invoices that people did not receive in the mail (or so they said). I'm sure some of them were valid because there were good regular payers who said this. There were others who are just consistently behind. Do you know how many I got back in the mail as just undeliverable? None. It doesn't surprise me at all. In that time period we also averaged $12,000-15000 per month in postage (We mail out a newspaper). Many of them didn't get delivered either because I had people I contacted to find out if they received them.
I'm not saying I'm a saint on this. I really need to do a better job myself. Most of my billing is different because our main revenue is printing and newspaper advertising. We will bill ahead when the ads come instead of at the end of the run cycle like we used to. This has helped alot. We also require certain ads paid for upfront, like our
signs.
If you don't have time to call, have someone in your office do it and do it regularly.
Hope this helps.