Some journals offer the possibility for authors to make the publication available by open access. This is quite expensive option but maybe you asked yourself, if it is possible to get more cites in spending some money. In a recent study (OpenAccess and Readership ) the number of citations of open access vs. regular access (via the library or repositories) was compared. And: it does not make any difference.
Spend you money for some other nice things!
ps: The most citations you get from reviews which is not unexpected.