Mountain weather is notoriously unpredictable and can change radically with little or no warning.
Always take extra clothing
Always take an extra jersey, wind- and waterproof outer clothing, gloves, and hat with you, even on the sunniest days.
Always use sunscreen and sunglasses, even on dull days.
Let other people know your plans
Always make sure your intentions are known to someone staying inside; where you are going, what you are wearing, and when you expect to be back.