Keeping a good diet, along with hydrating the body play an important role in remaining comfortable. A well-prepared packer should have sufficient clothing for any situation. If you do not have the experience to know what is needed, try your local outdoor store and ask for the hiking guru. This person can sometimes shed some light on what is needed in different hiking conditions. Learn from someone else's experience.
Books can help also.
Ground pads are necessary to determine a temperature rating of a sleeping bag. The pads acts as an insulation layer between you, your sleeping bag, and the ground. Ground pads are not just for comfort. Always purchase a pad with foam as core and not an air filled mattress. Air is a poor insulator and will continually pull heat from your bag, just as the ground.