A starting point would be to adopt a policy and stick to it - as the ACA has done: http://www.amercrystalassn.org/speaker-policy <http://www.amercrystalassn.org/speaker-policy> Similarly SCANZ: http://www.crystal29.com/conference-policy/ <http://www.crystal29.com/conference-policy/>
For more information on how to achieve gender balance at conferences/symposia/seminar series see: http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1003903 <http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1003903> Jenny
