Needed this answer myself and from the link provided by David Moye, decided on this and thought it might be of use to others with the same question:
CREATE PROCEDURE ...
AS
BEGIN
BEGIN TRANSACTION
-- lock table "a" till end of transaction
SELECT ...
FROM a
WITH (TABLOCK, HOLDLOCK)
WHERE ...
-- do some other stuff (including inserting/updating table "a")
-- release lock
COMMIT TRANSACTION
END