choosing the right locking mode

Given this type query:

         UPDATE bw_pool
         SET user_id=?
         WHERE bw_id=
                 (SELECT MIN(bw_id) FROM bw_pool WHERE user_id IS NULL)
         RETURNING bw_id

The idea is to "single-threadedly" get at the next available empty slot,
no matter how many such queries run in parallel. So far I've been
semi-successfully using LOCK TABLE bw_pool before the UPDATE, but it
deadlocks sometimes. Maybe I could use some less restrictive locking
mode and prevent possible collisions at the same time?

Thanks.