`C-u SPC` which pops the mark. This allows you to go back to different places in your buffer quickly. For example, if you are editing a program and need to add an import statement, you could do it like this: `C-SPC M-< [add your import] C-u SPC` and you'll be back to where you were editing.

If you are using transient-mark-mode, you can use `C-SPC C-SPC` to active and deactive the highlighted region. That'll still leave a mark.