If your current storage space is maxed out and you're still needing more, it might be time to consider the space between the studs. Christopher Patrick of Christopher Patrick Interiors demonstrates the way with custom recessed shelves he designed for an upstairs bath at the 2012 DC Design House.
Although variations may occur depending on the construction of your home, interior wall studs are typically spaced 16 inches on center with a depth of about 3 ½ inches . In addition to bathrooms, this space is sufficient to provide useful solutions in other rooms too. If you have a cramped entry way, consider a recessed space for coats, hats and umbrellas. A recessed wine rack allows for easy access for entertaining or a storage cabinet with doors conceals extra space for pantry items. The empty hallway wall space between bedrooms could be the new home for a recessed cabinet housing cleaning supplies or recessed shelves in a home office can provide orderly storage for desk supplies.
If the interior wall is not load bearing, it may even be possible for you or your carpenter to safely remove a stud to provide for wider storage. If you do decide to tackle this as a do-it-yourself project, make sure there's no plumbing, ductwork, or electrical wiring behind the wall. Nadia Subaran of Aidan Design created this wide glass cabinet built-in at the 2012 Design House kitchen. Love it!