If you're driving, no matter where you're coming from you'll have to take US 395. This is probably one of the most scenic highways in America: it stretches from up near Oregon (maybe even further) down the Eastern Sierra, eventually heading into the California desert. It is the only road that leads to Mammoth. Around Mammoth, US 395 has consistently spectacular and unbelievable views of the Eastern Sierra Nevada's escarpment. It also relatively easy to drive and wide.
The best way to get to Mammoth Lakes is by car, and it will be a scenic journey whichever direction you travel from.
If you are only skiing then you will be able to get around by the free bus shuttle to the mountain - this will also drop you off around town if you don't want to walk - It does finish around 6pm though.
Try Eagle Lodge before the Main or Canyon Lodge. If you get there earlier enough (by 9:00 a.m. usually) you can get a parking spot very close to the lifts.