1) It is always best to start early and arrive early. Make sure your bike is loaded and engine is warm so you can head out at the break of dawn. By doing this you will avoid a lot of traffic. More importantly, the water level in the water crossing/glacial melts will not be high and will be easier and safer to cross. It also gives you a lot of buffer time in case of a breakdown or landslide enroute.
2) It is important to have good quality waterproof riding boots. A good alternative is gumboots as it is ideal during water crossings.(use the light weight gumboots made of same material as crocks floaters). If water enters, empty it out immediately, wear a new pair of socks and tie the wet socks to your motorcycle luggage rack or handlebar to dry. The worst thing one can do is ride with cold and wet feet. Wet feet will lead to the body getting cold which in turn may lead to AMS.
3) Your luggage should always be wrapped in a waterproof cover or tarpaulin to prevent clothes from getting wet during rain or while riding through a glacial melt/nallah/water crossing.
4) Being a cold desert, Ladakh & Spiti can be very cold at nights and early morning. However post 8-9 am it can be extremely hot. Its best to wear thermal and your riding jacket inners when starting to ride. Once it gets hot you can take them off and put it your tank bag.
5) Try and carry as little luggage as possible. Carry a tent & sleeping bag only if essential and if you are going to camp in areas where accommodation is not available. It really does not make any sense carrying a sleeping bag and tent just to camp for a night, Also balance your luggage properly. Make sure the centre of gravity is forward and not at the rear of your motorcycle. Avoid excess weight at the rear as this will make the front end of your motorcycle jump up on even the slightest of gravel. Try putting most of your heavy stuff like tools etc in the tank bag/crash guard bags or side panniers.