Officially, the exit rows are only available at check-in. In practice, I find that by calling and simply asking for the exit row seats by name and not mentioning that they're exit seats, i.e. "18A", not "exit row seat 18A", I am given them 90% of the time. If it doesn't work, I'll mention that I am Star Gold, and that usually works. If not, hanging up and calling again usually does the trick. Certain call centres seem better than others at this.

Note that ET's website erroneously identifies the exit rows on the 738s as being rows 16 and 17, as opposed to rows 17 (exit row but no recline) and 18 as they are in fact. Therefore, you can snag seats in row 18 online with no problems.