A coat of primer and at least one coat of paint.
This protects the drywall mud from water and allows the paper to strip without removing the top layer of drywall.
Apply size befiore the wallpaper goes up.

All you need to do is make sure the walls are CLEAN (NO dust) and apply a Wallpaper SPECIFIC primer. The key is that it HAS to be a wallpaper primer,one good coat is all that is needed. I have been doing this for 20 years and stripping is a breeze when the walls are properly prepared. Forget (sizing) this is a term of the past and is not needed.I can and woukd be happy to explain further if necessary.

Bump....
I'm getting ready to wallpaper some new drywall. Our painter put primer on the new drywall, but I'm not sure if it was just paint primer or wallpaper primer.
I'm hearing conflicting information about doing both wallpaper primer and sizing or just one.
Can anyone clarify?
Christophern, if you see this could you explain further? And do you have a suggestion as to what brand to buy? Can I find it at HD/Lowes or do I need to go to a paint/wallpaper store?
Thanks!

Here is a link that should explain quicker than I can type

Shieldz is a good one
http://www.zinsser.com/product_detail.asp?ProductID=30

you should be able to get this anywhere they sell paint

Here is a link that might be useful: wallpaper

I have painted sheetrock in my hallway > should I put primer over the paint before hanging wallpaper or just install the paper over the painted surface.