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).