The first thing to consider is how much water you give your orchid; orchids naturally grow in exposed environments and it's easy to keep their roots too wet when they're potted. The most common mistakes that get made are either watering too often (and thus always keeping the roots wet) or not watering thoroughly enough (so the roots don't drink enough water).

One common way to water your orchid is to take it to the sink, remove the decorative pot, rinse all the roots with tepid water (even ones outside the pot) very thoroughly (10 seconds or so), and then wait about 1 week until watering again once the potting mix gets dry and light. If it's in a transparent pot, the roots should turn silver before watering.