Fit an extractor fan
This will rid the room of steam quickly. To find out what size fan you need, calculate your bathroom's volume in cubic metres (LxWxH), then multiply by 20 (the number of air changes per hour required in a bathroom). Get an electrician to fit it, so it meets building regulations.
Avoid papering areas prone to splashes of water
These are often the areas around the bath or basin. If you really want to hang bathroom wallpaper in these areas, fix low-iron glass on top to protect it - ask a local glazier to cut it to size.
After wallpapering, coat with decorator's varnish
This will protect the surface from steam and humidity. Polyvine sells a suitable product.