Teaching English - the vast majority of working holiday makers in Japan teach English. It used to be fairly easy to find a job as an English teacher, but due to the large numbers of foreigners coming to Japan seeking jobs in this field, it has become quite difficult. Without contacts it can take newcomers quite a few months to secure a teaching position; all the while Japan's high cost of living will eat away at your funds. This is especially so in Tokyo. A TESOL (Teaching English to Speakers of Other Languages) is essential. Many Australian companies now offer this qualification and usually operate a job finding service also. Unless you have a TESOL job lined up through your own contacts this is the best way to find teaching work in Japan.

The chances of finding a job in specialized areas such as designing, landscaping, hairdressing, etc. are quite limited. The fact that you have a specific skill or degree may have no bearing on what kind of jobs are available to you. On the other hand, manual labour work in restaurants and hotels can be quite easily obtained, especially if you have some Japanese ability, though the wages are relatively low (on par with the standard wages of young Japanese workers, i.e. ranging from 650 yen/hr to 1,000 yen/hr). 
Ski resort jobs are very popular amongst the Japanese as well as foreigners. These jobs are very hard to get and usually require being in the right place at the right time and a lot of luck. Modelling jobs are also available, but contrary to popular belief, they are not handed out to any and every Westerner. Experience and a portfolio are usually required. Live-in and au pair jobs are very uncommon in Japan because of limited space and the fact that most Japanese consider their homes very private.

There used to be a limit for working hours, but not now. Also you are permitted to do any type of work that you wish except to work at bars, nightclubs, hostessing/host clubs, gambling establishments or any other similar premises.