The key is to have the camera absolutely perpendicular to to the ad after putting it flat against the wall. Stick a small mirror on top of the ad and tape two threads making a cross hair on top of that. Position the camera in front until the ad fills the LCD/VF, then fine tune by shifting the camera on the tripod until the lens in the mirror sits on top of the crosshair right in the center of the LCD/VF. Remove mirror. Use indirect light, paper works as a diffuser, white cardboard works as a diffusing reflector. Make sure the ad is evenly lit, use two light sources left & right or maybe one from below. Color balance the camera on a 18% gray card on top of the ad before the shoot. Use the biggest aperature possible (smallest f-no.) and lowest ISO available for best results.
Edit: don't use the build-in flash. If you you have an external flash you could try it w/ a diffuser way off the camera.
Borders? I have never seen one. But I have heard they exist in the minds of some people. -- Thor Heyerdahl
Durch einen Stich bereits geschafft, erschlafft und ohne Saft und Kraft! -- Donald, examining a Deflator Dextrospirillus