When you enter Japan, you will probably be on a 90 day tourist visa exemption. Holders of certain passports (e.g. Ireland) can get this extended by an extra 90 days. You must leave the country before your exemption runs out, but you can return straight away and get another 90 days. Some people worry that the the immigration people will eventually get suspicious if you keep doing this, but I have also heard of people who have done it five times with no problems.
To get a work visa, you need to find a company willing to give you a job. You will also need the appropriate qualifications. If you have a four-year degree from a reputable university, that should be enough. If you have anything less then things will be more difficult. For example, if you have a 2-year degree or diploma, or an MBA from an institution nobody has heard of, then you and your potential employer will have to argue your case with the immigration department.