Type 'dmesg' just after plugging it in to see what device it is assigned (/dev/sda very often). Add a line to /etc/fstab like:
/dev/sda1 /mnt/usbdrive TYPE noauto,user 0 0
(substitute the filesystem type -- vfat, ext2, ext3, etc. -- for TYPE.) and do a 'mkdir /mnt/usbdrive'. Then you can 'mount /mnt/usbdrive'.