What she said.
Could be other issues, but most common would be the humidity issue (or lack thereof). You can also stick the whole plant in the shower every now & then and just let the water run over it (pot & all) for 10-15 minutes. This helps wash out build up of minerals & salts from the tap water that can eventually cause problems. Plus it gives the plant a nice bath to get any built up dust & dirt off the leaves.
