How to mount an USB drive maually so everyone can read/write to it?

Not only the umask=1000 but also users (plural) for more than one user and rw (read/write).

Read the man pages for umask, mount, fstab and this Quick and Dirty Guide to Linux File Permissions

In /etc/fstab:
Code:
/dev/sda1        /mnt/flash       vfat        users,umask=1000,rw,noauto 0   0

Then run:
mount /mnt/flash