There are generally two types of bubbles associated with hanging wallpaper which are either due to trapped air or more seriously, continued expansion.
Air bubbles can generally be identified by their shape and will show themselves as a relatively 'round' type bubble (diameter approx 4-8 inches). These are caused when the paper is smoothed unevenly. The best way to avoid air bubbles is to gently brush the paper down the centre and then out to the edges as you work down the sheet. If bubbles are immediately identified as the wallpaper is being hung then simply lift the wallpaper from the bottom and re-brush/smooth it back down. Air bubbles will reduce slightly upon drying but will not disappear completely. Pricking them with a needle and then attempting to brush them out after they have dried will almost certainly result in creases.
Expansion bubbles or creases are caused by the continual expansion of the wallpaper. This occurs when the paper has not been left to soak for the recommended time. They normally present themselves as short elongated bubbles (approx 10/15cm long), running parallel to the length of the wallpaper and tend to look staggered across the sheet. It is highly unlikely that these type of bubbles will reduce upon drying or be removable once dry.