dd if=</path/to/file> of=<device> bs=4M; sync

where:

<file> is the name of the input image, e.g. "netinst.iso"

<device> is the device matching the USB flash drive, e.g. /dev/sda, /dev/sdb. Be careful to make sure you have the right device name, as this command is capable of writing over your hard disk just as easily if you get the wrong one!

"bs=4M" tells dd to read/write in 4 megabyte chunks for better performance; the default is 512 bytes, which will be much slower

The "sync" is to make sure that all the writes are flushed out before the command returns