I love making smoothies and have noticed that after a few hours the
juice and rest of the ingredients separate.  What preservatives
(preferably natural)and in what quantity should I add preservatives to
the smoothie so that it will not separate and will last at least a
week.  I want to put these smoothies in plastic bottles so will this
make a difference?

gelatin might work

Have you tried using lecithin as an emulsifier?

I sort of have an idea of what to put in but not the quantities. 
Thats really the answers I was looking for, would be appreciated if
you could help on that point.