I have a system.  It's called "if this room is so dirty that I can't stand it anymore, it's time to clean it" :)  I really clean at random depending on what looks dirty or what's been annoying me - like if the mail starts to pile up, one day that just annoys me and I go through it all.

It's amazing how little things being done here and there will add up to a house that looks pretty clean for the most part.  My friend was coming over last night and we were a bit messy.  I took 5 minutes putting away some blankets and dirty glasses and was amazed at how my living room looked spotless.