Ordering Citations in LaTeX by Appearance

Change

\bibliographystyle{plain}

to

\bibliographystyle{ieeetr}

Then rebuild it a few times to replace the .aux and .bbl files that were made when you used the plain style.

If you use MiKTeX you shouldn't need to download anything extra.

Note: 'plain' style uses alphabetical order.

source: http://stackoverflow.com/questions/144639/how-do-i-order-citations-by-appearance-using-bibtex