Avoid Cotton: As always, avoid cotton or cotton blends as they will just get wet and stay wet. With pants, especially, you can set yourself up for catching a chill or even hypothermia by wearing jeans in the rain.
Rain Pants: If your walks are likely to be in the rain, you may want to invest in a pair of waterproof rain pants. These can be expensive, but can also provide a great deal of comfort. They are also windproof, and so can do double duty if you are in a windy climate. Some double layer waterproof pants have an inner fleece layer for very cold climates or skiing. That may be too much insulation if your walks are above 30 F.