The main things I put on the floor are trousers and skirts.  I refuse to pull the mannequin apart for trousers and risk damaging her (she has to be taken off the stand to thread the legs over her feet).  I can't seem to get a good result with hanging trousers either, they just look better on the floor.  

Some skirts are the same too.  I don't think they come up well on the mannequin because of the way she kicks her leg out, unless they are really long.  Some skirts, like jersey, are too drapey to put on a hanger, but come up a treat on the floor.  

I used to have a wooden floor before I moved, really miss that.  All the clothes looked wonderful on the floor then, carpet is a bit bleh.

If you keep a lookout you can score a mannequin for under $100 delivered.  I think the flesh coloured ones are the best because you can see light coloured clothes on them - I've been discussing with my partner for awhile now about re-spraying mine (he's got a spray booth).  My other advice would be don't get one with a full made-up face and hair, or at least crop it out of your photos.  They just look creepy.  

Other than that, take a good look at how other sellers present their clothing, you can learn alot.  Not having a mannequin doesn't mean you won't do well.  A few prominent sellers just hang everything and do well, some that use mannequins look dreadful.