These are usually the easiest spaces to design since they are small, but they are often left out anyhow.  There are three classes of storage: cabinets, closets and bulk storage.  Historically houses had no closets at all, and instead used various cabinets (for example armoires instead of bedroom closets).  This solution has the advantage of flexibility, and because the walls are thin, it takes up less floor space, but it generally more expensive than a closet.

Every room that needs storage should be designed with the storage as part of the room; not just the common things like kitchen cabinets, but pantry space, a place for the vacuum, recycle, cloth bags, spare towels, and all the other things that don't have specific traditional storage places.

There are standard solutions to some storage needs, such as bedroom closets, which often sit in a thickened wall between two bedrooms.  A less traditional solution is to thicken halls and put cabinets or bookshelves in the space.  In general cabinets are more space efficient than closets because they have thinner walls, and so one option is to build closets as cabinets instead.

Storage can also go in other non-traditional spaces: underneath stairs, under a window seat, under a platform bed and so on.

When designing storage space, keep in mind that once a shelf gets more than about 12" deep, you can't get to the items in the back because there is too much in front of them.  Larger items like pots can be in 24" deep cabinets and still never been more than two deep.  Clothes and coat closets need only be about 18" deep--large enough to easily accommodate clothing on a hanger. On solution is to use thickened doors with shelves on them, another is to put shelves on full extension drawer glides.

Walk-in storage is less space efficient than cabinet storage, because the walkway between the two sides stores nothing.  Obviously if you have to make some space bigger to accommodate cabinets, than there might not be a space savings.

Dressing room:  One way to expand the master bathroom space is to make a walk-in closet/dressing room adjacent to it.  Having the two combined will tend to make fitting them harder than having them separate, but if you like the idea, it doesn't hurt to try.

Bulk Storage - Almost everyone own things they don't use often (and many own things they don't really need), holiday decorations, sports equipment, spare chairs, and all the other items that show up at garage sales.  Basements are good places for some of these, but make sure the basement stays dry.  Attics are better, although if they're unheated, the temperature range in an attic can be quite large, and in particular attics can get quite hot: 120°F is not uncommon.