You know there's a setting on your list that you can set it to immediate payment only. Means they have to pay asap. I don't know how long they have though but all my buyers pay within minutes of ordering
Adjust your settings so unpaid cases are opened automatically after a given time. I allow 4 days then a case opens automatically. If they still don't pay, the case is closed automatically and you receive your fees back after a further 4 days.
If you do auctions then you're screwed but you are screwed in anyway if still doing auctions on ebay, if BIN then as others have said change your settings to instant payment and then if they do not pay instantly then the item is not changed to sold.
Also make use of your buyer requirements when ever possible, if you don't already as it stops some of the idiots.