Make sure that there's nothing extra on the page (page numbers, for example, using `\thispagestyle{empty}`) and then use the command `pdfcrop` which is included in TeXLive.  (It's a perl script).

That's what I used to make this picture for [this answer](http://mathoverflow.net/questions/90/fn-2n-is-not-surjective) on MathOverflow:

<img src="http://www.math.ntnu.no/~stacey/myicons/2n_graph_small.png" alt="Graph of n to 2n" title="Graph of n to 2n">

(source file: <http://www.math.ntnu.no/~stacey/myicons/2n_graph_small.tex>)