Get your bike fully serviced by a competent mechanic at least 3-4 days before going to the hills. Uphill roads put a lot of pressure on the engine, so changing the engine oil is mandatory. Also check the brake pads, clutch cable, chain set and spark plug.
Good grip on hilly roads is important. If the thread or groove of the bike tire is worn, replace the tire before the trip. High wind or low wind in the mountains - both are dangerous.
Maintain the manufacturer's recommended tire pressure when riding on hilly roads. Mountain fog can fall suddenly or rain can start at any time. So the headlight, indicator and pass-light of the bike should be in perfect condition.
Horning is mandatory in hilly turns (Blind Turns), so it is very important to have a strong and active horn. If necessary, you can install additional fog lights. It is a matter of luck to find a mechanic on a deserted mountain road.
So carry a puncture kit, mini air pump, spare clutch cable, brake cable and a spark plug with you. It would be wise to learn the basics of how to use them before you leave. Before you leave, find out about the weather on the route you are going and whether there are any landslides or road closures through local news or social media.
It is best to avoid mountain bike riding as much as possible during rainy or heavy snowfall seasons. Prioritize safety over style in the mountains. Use a good quality full-face helmet, riding jacket, gloves and knee guard.
Mountain weather can change quickly, so be sure to pack a good raincoat and waterproof shoe covers along with your riding gear. Carry all valid bike documents like RC Book (RC), Insurance, Pollution Control Certificate (PUC) and your driving license. Several hilly border areas like Ladakh, Arunachal or Sikkim require special Inner Line Permits (ILP), procure them in advance online or offline.
Always follow 'lane driving' in hills. Don't be in a hurry while overtaking and don't forget to enter the other lane at the corner. Remember, returning safely is more important than the joy of reaching your destination.