Something approaching 15% of all peer reviewed periodicals are open access. The Directory of Open Access Journals (DOAJ) now lists 3009 journals meeting their selection criteria. There are not yet enough open access periodicals in ancient studies to suit my taste, but more are appearing all the time. Have a look at the DOAJ Subject Tree for a good view of what's available there.
Push your scholarship towards open access journals. Encourage your students, colleagues and teachers to publish in open access journals. Push postprint copies of your already published work to your institutional repositories.
If you have open access pubications, make sure you bring them to the attention of those people, like me, who maintain portals to or indexes of open access material. If you have open access publications on the ancient world, let me know, and I'll index them in Abzu.
And one more thing. Look at what institutions like The Oriental Institute, University of Chicago, are doing - simultaneous publication on paper (for sale) and in online digital facsimile (for free), and working through their back list of both in print and out of print publications with the goal of building a complete set. Encourage other institutions to develop and implement comparable policies.