There are institutions that can assist you in distributing the metadata about your collection to a larger audience.  It is like getting free advertizing.

Harvesters can have a hard time finding out about your collection, so it is best to contact them and inform them of the availability of your metadata.  You can begin by validatiing your OAI installation and registering your site at ...

http://www.openarchives.org/Register/ValidateSite

If you want to be proactive, you can contact service providers (Harvesters) directly.  Here is a handy list of service providers from the Open Archives Initiative site...

http://www.openarchives.org/service/listproviders.html