On becoming an ACE
You could maintain a compelling blog like Mark Rittman. You could single-handedly create a tradition of best practice in PL/SQL programming like Steven Feuerstein. You could become the leading expert in installing Oracle on Linux like Werner Puschitz. You could run an excellent community website, give deep answers to questions on Oracle listservers and write a book on the Cost-Based Optimizer like Jonathan Lewis. Being an OCM didn't do Laurent Schneider any harm. Or, of course, you could just be as knowledgeable, dedicated and evangelical as Tom Kyte.
In that light, becoming an ACE for just answering questions in the OTN forums looks like the easy option. And it is. All that is required is patience with newbies, time to read and research questions, a basic level of writing ability and a willingness to engage across a variety of topics. Enthusiasm for good programming and administration is important. A sense of humour can be useful (but also dangerous: not everybody will get the joke). It also helps to know what you're talking about although I've never let this hold me back ;) For what it's worth I wrote on how to be a good guru some time ago.
In the end, you can't set out to become an ACE. It's something that happens to you along the way, a sign that somebody else has spotted your efforts and deemed them worthy of merit. Nor should you bother to try to become an ACE. If the activity is not meaningful to you irrespective of ACE-hood then you probably won't make the grade anyway. The perks are not that great (a complementary pass to Oracle Open World but meet your own expenses and that cool ACE logo whenever you post in the Forums). But the recognition is priceless. In this respect it's not unlike a janitor in the MoD being given the MBE for thirty-five years of service (i.e. cleaning toilets).
The bar has certainly been raised since I was nominated. If I look at some of the people who are now ACEs I think I must have walked into the wrong club. But I am an ACE too and I've got the sweatshirt to prove it.