O2 advice apart, one of the main things to plan for carefully is not just the weather but specifically the presence of wave conditions. In the area of the Alps I fly a lot the active wave areas are fairly predictable for different wind directions, but that does not hold true for all alpine areas. Even well above mountain tops one can get caught in heavy sinking air (20 kts plus) and getting into wave rotor can be like being inside a tumble dryer with total loss of control. It is extremely violent and frightening for the uninitiated. e.g in a N/NW wind, to the south of the 10,000 ft Pic de Bure which is NW of Gap (France) in the Durance valley.
So when you plan your route look specifically at weather data that provides forecast wave conditions and avoid that like the plague, at least the down parts of the wave or the areas of rotor below the wave contact points.
It's different for me in that as a (mainly) glider pilot with many alpine hours I often seek the wave in the mountains, but one has to be very cautious, and in a powered aircraft it is a different ball game to a high performance glider. Hope that helps.