easy task: concurrent select-updates

Hi All,

I have a trivial task. There is a table with messages queue, let's say
"msg_queue".
There are a few processes and each of them is taking one message from
this table at a time to transmit into communication channel.
I've done it my way, but I have postgresql's messages about deadlocks
and a lot of warnings.

I my program, every process is doing approx the following procedure:
SELECT ... FROM msg_queue WHERE busy = false ORDER BY ... LIMIT 1;
if a message was found:
BEGIN;
SELECT id FROM msg_queue WHERE id = ... AND busy = false FOR SHARE;
UPDATE msg_queue SET busy = true, channel_id = ... WHERE id = ... AND
busy = false;
COMMIT;

I do understand that this way is stupid, but I have not came with
anything else yet.
Could somebody share ideas how to do this so the same message 100% WOULD
NOT be transmitted over two or more channels.
Sorry for the newbie question!

Best regards, Nick.