Puffin, loads(!) of useful advice here already. I'll just add my bit to it.
Each week I used to to pack every inch of a Honda Civic with huge amounts of clothing and bric-a-brac, plus stall rails attached to the roof, for a market stall I ran. Leading on from this, here are some general tips:
Small and/or squishy bags are a must.
Pack the biggest and most awkward items first. Pack small items around them.
Keep shoes loose - they can tuck in anywhere. Pair them up on arrival.
Think hard about the number of jumpers, fleeces and coats you need - and keep them loose again. Lay them on top of the packed luggage in the boot, just before you slam down the door.
For safety's sake, sit in the driving seat every now and again as you load up, just to check that the the driver can still see out of the rear view mirror.
When it comes to loading the passenger compartment, squash in more by keeping all doors closed, apart from the one you're using to load in luggage.
When you get to the last few small items, get everyone seated first, then tuck the things around them on the seats and in the footwell.
Never, ever pack ánything in the driver's footwell. There is always the danger of items getting caught in, or jamming the pedels while you are driving.
Most importantly: Pack your bag of journey essentials last, and within easy reach!!