The IEEEtran bibtex style has an interesting mechanism for customizing the bibliography. It is documented in IEEEtran_bst_HOWTO.pdf, see IEEEtran bibtex on ctan.
You create a special entry in the bibliography file, of type @IEEEtranBSTCTL, where you can adjust various parameters for the bibliography style. Amongst these, the "text" printed before url fields is controlled by the field CTLname_url_prefix. By default it is "[Online]. Available:".
So one possibility is to change that text to start with the command \par, starting a new paragraph. An alternative would be \linebreak{}, which would attempt to stretch the previous line to the right margin. (The extra braces {} would be needed because af the following square bracket at [Online] in this case, which should not be interpreted as an optional argument.)
Now you need to cite this styling entry before your other citations in the main document via the special command \bstctlcite.
Here is an example:

\documentclass{IEEEtran} \usepackage{url} \begin{document} \bstctlcite{IEEEexample:BSTcontrol} Text with citation~\cite{test}. \bibliographystyle{IEEEtran} \bibliography{IEEEabrv,ref} \end{document}
with ref.bib containing
@IEEEtranBSTCTL{IEEEexample:BSTcontrol, CTLname_url_prefix = "\par [Online]. Available:", } @InProceedings{test, author = {Chamseddine, A. and Tong Li and Youmin Zhang and Rabbath, C.-A. and Theilliol, D.}, booktitle = {American Control Conference (ACC), 2012}, title = {Flatness-based trajectory planning for a quadrotor Unmanned Aerial Vehicle test-bed considering actuator and system constraints}, year = {June}, pages = {920-925}, issn = {0743-1619}, url = {http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6315362&isnumber=6314593} }