Here's one keybinding I could not live without.

(global-set-key (kbd "M-j")
            (lambda ()
                  (interactive)
                  (join-line -1)))

It joins the following line onto this one.

Let's say I want to collapse this paragraph-tag to one line:

  <p class="example">
    Some text
    over multiple
    lines.
  </p>

With point anywhere on the first line, I simply press M-j multiple times to pull the lines up.