M-\\ (M-x delete-horizontal-space) removes all spaces between non-spaces.

But this key binding is not convenient to strike and remember...

There is a similar key binding M-SPC (just-one-space) - remove all spaces and add one.

In 2013-01-30 Michal Nazarewicz introduced a new function cycle-spacing that combines both just-one-space and delete-horizontal-space by cycling between one space, no space and original spacing.

Now I have in my .emacs:

(when (fboundp 'cycle-spacing)
  (global-set-key (kbd "M-SPC") 'cycle-spacing))