Route selection, considering weather, terrain, a/c performance and equipment, oxygen requirements...
Kinds of mountain enroute cruise
Climb to 2000 above the nearest or highest terrain, set sail. Or;
Through and between valleys and mountains
Compute ground speed in-flight. Guideline: for a climb leg, add 1 minute for each 1000 ft of climb.
File and maintain a flight plan - best free insurance.