Create partition on your flash drive (using fdisk or cfdisk). Mark one of partitions bootable in fdisk/cdisk. Format that partition with FAT/FAT32 filesystem:
mkfs.vfat -F 32 /dev/sdXY
where sdXY is your partition.
Make MBR record:
ms-sys -s /dev/sdX
(where sdX is entire flash disk; ms-sys comes from ms-sys package)
syslinux -s /dev/sdXY
to load syslinux onto your flash drive.
Reboot your system and check if it boots correctly 🙂
Note that some systems have problems with booting from flash driver (especially big like 1G or 2G flash drives).