Base layer consists of thinner garments with high wicking properties to prevent moisture being felt on the skin e.g. polypropylene top. Mid layer generally consists of fleece and fibrepile garments. This is the layer that does most to keep you warm. Though it is important to remember two thinner layers keep you warmer than one thicker layer due to the air trapped between them. Jeans should not be worn since when wet they provide no insulation.
Breathable fabrics improve comfort levels but are more expensive.
Waterproof trousers while not essential do help to keep your legs warm and dry. Gaiters help to keep heather, mud and snow out of your boots and are essential in winter. In winter mittens are better than gloves because you can move your fingers inside them though they are more awkward. Ski goggles also can be useful. In summer sun hat, sunscreen and sunglasses are advisable.