The first possible cause of a sewage smell in your bathroom is by far the easiest to fix. If your bathroom fixtures haven't been used recently (the sink in a guest bathroom or the tub in a bathroom with a separate shower, for example), it could be that the water in the P-trap has simply evaporated, removing the barrier between your bathroom and the gases in the sewer system. If that's the case, the solution is as simple as running some water to allow the P-trap to fill up again. If you smell the odor in a bathroom that's being remodeled, be sure to cap any open drain pipes until your new fixtures are installed.
If there's still a sewage odor after you've run plenty of water down all the drains, or if you notice a sewage smell in a bathroom that gets regular use, you might want to check for leaks in or around the P-trap. If even a small amount of water has leaked out of the bottom of the "U," sewage gases may be able to sneak in. Another possible cause that's kind of gross but fairly easy to resolve is a clog in the drain, since the clog itself can become smelly.
If the suggestions above don't lead you to the source of the sewage smell, you may be dealing with a more complicated issue like the ones on the next page.