During the ski season, Chūō Bus coaches run from Sapporo Eki-mae Bus Terminal (¥2240/4000 one-way/return, three hours) and New Chitose Airport (one-way/return ¥2600/4500, 3¾ hours) to Niseko; travel times are dependent on weather conditions. Drop-off points include the Welcome Centre in Hirafu, the Hilton and Annupuri. Reservations are necessary, and it's recommended that you book well ahead of your departure date.
Car & Motorcycle
Scenic Rte 5 winds from Sapporo to Otaru around the coast, and then cuts inland through the mountains down to Niseko. Having a car will make it easier to move between the various ski slopes, though you'll need to drive with extreme caution. In summer (low season), public transport services drop off, which provides more incentive to pick up a car in Sapporo or at New Chitose Airport. There are also outlets of major car-rental companies in Hirafu.
The JR Hakodate line runs from Sapporo to Kutchan (¥1840, 2½ hours), Hirafu (¥1840, 3½ hours) and Niseko (¥2160, three hours) stations. You will need to transfer in Otaru, where you can also get trains directly to New Chitose Airport.
Note that JR Hirafu Station is actually inconvenient for central Hirafu, unless you've arranged for pickup.