How to order citations by appearance using BibTeX?

Question

By default (using the plain style) BibTeX orders citations alphabetically.

How to order the citations by order of appearance in the document?

Answer 1

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.

Or simply delete the .aux and .bbl files and rebuild.

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