Formatting back references in bibliography [BibTeX]

Use:

\usepackage[pagebackref=true]{hyperref}
\renewcommand*{\backref}[1]{}
\renewcommand*{\backrefalt}[4]{[{\tiny%
    \ifcase #1 Not cited.%
          \or Cited on page~#2.%
          \else Cited on pages #2.%
    \fi%
    }]}

I put this is in a \tiny font in between square brackets []. Change to your own tastes:)

For a similar scenario, I preferred using a more concise text and explain it at the beginning of the bibliography, rather than using \tiny (which looks bad here, especially inside normal-sized brackets). Combining Andrew's answer with natbib's \bibpreamble and some space before the actual reference list with \bigskip, I ended up with the following setup, to customize as needed:

% Add any needed options:
\usepackage{natbib}
\usepackage[backref=page]{hyperref}

% Customize list of backreferences.
% From https://tex.stackexchange.com/a/183735/1340
\renewcommand*{\backref}[1]{}
\renewcommand*{\backrefalt}[4]{%
    \ifcase #1%
          \or [Page~#2.]%
          \else [Pages~#2.]%
    \fi%
    }
% Explain list of backreferences.
% https://tex.stackexchange.com/a/70953/1340
\renewcommand{\bibpreamble}{%
  [Citing pages are listed after each reference.]%
  \par\bigskip}

Of course, one can customize that preamble text (and formatting) further. To move the prelude closer to the title, for now I've settled on the following—but I'm not sure such low-level tuning is good typography (the above looks safer), so YMMV:

\newcommand{\myBibPreambleText}{[Citing pages are listed after each reference.]}

\renewcommand{\bibpreamble}{%
  \vspace{-2 \bigskipamount}%
  \myBibPreambleText%
  \par \vspace{2 \bigskipamount}}

EDIT: instead of hacking vertical spacing like that, it's probably more robust (typographically) to modify \bibsection to add a "subtitle", so that TeX computes the appropriate position. I can do this with the following code, where I've added \\ {\normalsize\mdseries \myBibPreambleText} to the definition of \bibsection which was in use for me, unless I load the tocbibind package, which rewrites the relevant code.

\newcommand{\myBibPreambleText}{[Citing pages are listed after each reference.]}

\makeatletter
\renewcommand{\bibsection}{
  \chapter*{\bibname \\ {\normalsize\mdseries \myBibPreambleText}}
  \@mkboth {\MakeUppercase {\bibname }}{\MakeUppercase {\bibname }}
}
\makeatother