How can I set the order of membership organisations in dropdown?
While Membership Types can be ordered at civicrm/admin/member/membershipType it seems like the Organizations are sorted alphabetically here (CRM-6099).


You could rename the organisations to rearrange their alphabetical ordering. This is probably not realistic :)
You could create a CiviCRM extension which implements hook_civicrm_buildForm() and rearranges the options in the Organizations list.
A patch to core could make the form respect the order defined at civicrm/admin/member/membershipType for the related Organizations also.