If you just mount the usb stick without special options it is only read/writable for root.
To read/write enable the stick for a different user use the following:
mount /dev/sda1 /mnt/usbstick/ -o uid=500
User ID 500 is on many systems the default ID for your first user. You might want to switch it to another user (take a look at the ID in /etc/passwd) or set permissions for a group. For a group use "gid" instead of "uid".
It also might be handy to add a line like the following to your /etc/fstab file (where you can specify some default mount information):
/dev/sda1 /mnt/usbstick vfat noauto,users,exec,rw,umask=000 0 0
This allows the members of the group "users" to mount the device /dev/sda1 (which represents the usb stick on my system) with read and write access. Another advantage of this line in your /etc/fstab is that for some graphical environments (e.g. KDE) read the file at startup and present you a corresponding icon to mount and unmount the device automatically on your desktop.
