An additional tip...

 If you do make 'eyeballs' using fimo, it is quite handy to make them and before they dry hard insert the sharp end of a needle or pin into the pupil of the eye. This sounds gruesome but it enables you to easily adjust the position of the eye in the plasticine 'socket' to give nice effects like rolling of the eyes or looking from side to side.
 

if it's going to be a fairly long animation, i'd recommend making stuff like jumpers from fimo clay...i've found that with moving characters sometimes i've put fingers through people...not fabulous, really. ( :

but plasticine is definitly the best material you'll get for claymation.

 By 'Jumpers' I assume you mean sweaters, wooly pullovers or clothing in general...

 Just for the record if you want a description of Clay Animation or Claymation then you can find a very good description on Wikipedia.

yepp...things that won't be moving.

...it's how they do wallace and gromit, don't you know.