What is the best way to make a staircase? I have built a Bat Cave from some bookcases and lego base plates. ( http://alice-yarns.co.uk/blog/2013/11/17/the-batcave-part-i/ ) I want to add staircases for the characters to move between each of the three shelves. But what is the best way of doing this?

I wondered about making it purely from lego bricks but I can't see how this could be stable.
Building a staircase out of bricks is easy enough, but uses a lot of brick pieces, as zovits already answered. If you want to achieve something faster, with fewer pieces, you could look into the spiral-staircase-piece



or the regular staircase piece.



The spiral staircase piece combines with some plate+pin pieces (bottom, top) and some posts. You can connect as many posts together as you want using a pin. 

The advantage of using the spiral staircase is mainly the simplicity of it; it's not that versatile a piece, but it works for its purpose and does have a nice round edge. Plus the stairs can be rotated, which allows for a "hidden staircase" feature. 

The advantage of using the regular staircase piece is that it is very sturdy and it takes up very little space. You can build a similar staircase using the inverted slope and some 1x4 plates but it isn't as strong (and it's slightly taller).