When you let them out for playtime, lay on the floor and let them "explore" you at their leisure. They are curious little critters and will soon see you as their favorite playtime toy to climb on, and it will build up their trust in you a lot quicker.
Also, giving them access to their cage during playtime helps a LOT. It gives them a measure of security to be able to run in and out of their cages on their own when they are feeling skittish. If you have a Ferret Nation or Feisty Ferret, doggy steps fit really well up against the bottom door. If it is a smaller cage not on wheels, just set it on the floor during playtime and open the door. (in a chin-proofed room, of course.)