I can't speak to cleaning a live fish specifically for sushi/sashimi,
but I've cleaned plenty of fish in my time (live and dead) and I've
done it the same way. Some of the fish I cleaned was used as sushi and
I was never shown a special way to prepare it (I used to work in a
seafood department). I would think that the most important thing would
be to start with a clean, sanitized work area and make sure that
everything used in the cleaning, storing and serving process were
clean. A sharp knife is also important. Depending on the particular
fish, scissors or a glove may also help (see below). With a live fish
your are going to have lots of blood and will need a lot of water.

When cleaning a live fish, the first step is to stun/kill it by hitting
it in the head several times with a heavy object. Some sporting
goods/fishing supply stores sell specialized clubs but anything
(rolling pin, smal bat etc.) will do. You're aiming for the brain,
which is usually a little bit behind the eyes (but don't bash its head
to an unrecognizeable pulp). When the fish is stunned (you'll know
because it will stop flopping, although if you don't stun it hard
enough it may start flopping again) you can clean it like you would any
other fish. Assuming this is for sushi/sashimi, you'll be making
fillets. Once you've got the fillets be sure to wash away any
blood/bile/scales.

The glove will help you grasp it if it is a particularly slippery
species and if you are cleaning a fish with spines, you may want to cut
them with the scissors.

-FM