Here is a solution which works with biblatex independent of the backend used. Since the OP asked about using bibtex, my example specifies this, but nothing depends on it. backend=biber would work at least as well.
This answer is based on moewe's solution. I ran that code and read the output. biblatex suggested the defernumbers option there. Looking in the documentation, I learnt that defernumbers, unlike \DeclarePresort is not specific to biber. An added (but extremely minor) bonus is that it gets rid of the warning from biblatex on compilation.
In general, biblatex assigns numbers to reference entries at the beginning of the document. If defernumbers is set to true, however, it does not assign numbers until they are first used in the bibliography.
\documentclass[a4paper,12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage[sorting=nty,defernumbers=true,backend=bibtex]{biblatex}
\usepackage{hyperref}
\addbibresource{biblatex-examples.bib}
\begin{document}
\nocite{baez/online,cicero,wassenberg,wilde}
\printbibliography[nottype=online]
\printbibliography[type=online]
\end{document}

Note that the documentation warns that you need two runs of latex to get the numbering right, over and above the runs needed to get pagination etc. correct. Moreover, should you change the value of defernumbers, you will probably need to delete the .aux file. (Though I didn't find I needed to do this going from the default false setting to true.)
numericstyle ofbiblatex? Really, an MWE of even the barest kind of set-up you already have could help us. – moewe Mar 06 '14 at 10:23\DeclarePresortis biber only). Biber is the recommended backend now anyway. – moewe Mar 06 '14 at 14:04