Put all your existing paperwork into a box (or boxes!) and don't add any more to the box.  As new paperwork comes into the house, sort through and file it straight away.  When you have a few minutes to spare, take a few papers from the top of the pile and sort through them.  Don't worry about the rest until you have another few minutes when you feel motivated to do some more.

Don't plan to do more than, say, fifteen minutes' sorting at a time.  If you happen to spend a couple of hours sorting through, that's great, but only ever plan to do it in small, manageable chunks.