reorg of a tablespace

hi oracle guruus,

i'm running in a problem, i can't solve by myself. here's the subject:

we've created a tablespace with a datafile of 100Megs. after a year we've seen now, that 20Megs will be enough. (please no flames, we've had our reasons to size it this space, but they've  changed)
well, now we want to reduce the size. (need the diskspace) we've tried a full database backup, recreated the database with the new sized datafiles and tried to import it again. but then we've got an error with a different tablespace; the size we created it was even bigger than the original. the error was : unable to allocate extent .......

now my question:
1. what is the best way to reduce the size of a datafile in a tablespace???

2. why did i got the errors on a tablespace we sized up??

any help is welcome

thanks Peter

--
Peter Kesch             Kapp & Co                  email:kesch_at_kapp-coburg.de
phone:+49 9561 866376   Callenberger Strasse 52-58
fax:  +49 9561 866103   96450 Coburg, Germany