1

The only way you are going to get a (legal) job is to have a proper visa. Work visas require employers to sponsor you, and immigration has its requirements, too (usually a degree, so you probably don't qualify for that).

Your best option is to apply for a working holiday visa (if you haven't already used it in the past). The WHV lets you do practically any work, FT or PT, and you don't need an employer to sponsor you. The trick is to find someone willing to hire a 19-year-old with (presumably) weak Japanese skills. Bar work? Maybe, but drinking age here is older than you. Don't book until you know what sort of work you can get.

2

Yeah, I had a friend that worked in Niseko in the office of an accomodation booking company, but she was a 25 year old with real good Japanese. I'd worry about finding work. I think there are probably ski/boarding forums that are probably going to be more able to help with this question.

Sorry to be a downer too, but having lived in Sapporo and been into boarding, and friends with people who boarded, there were many conversations about quitting our English teaching jobs and doing something that would give us more time on the snow, and none of us came up with any great ideas. Hospitality would be the go I reckon.

3

Niseko hires a lot of people, especially Aussies with working holiday visas (if you're eligible).
Check out this site - they post up jobs and accommodation available, and you should email all the companies for vacancies. http://www.powderlife.com/
Not all jobs require Japanese, especially if you don't mind doing hospitality work and/or helping out at a lodging (doing cooking, cleaning, running a lodge etc). Most will offer board and lift pass.
I worked there for one winter season, loved it so much, I ended up staying a year and a half!