As far as I can tell from my reading, a common method for determining a time series' trend is to smooth the series, perhaps in an iterated fashion, as in:
[SBP paper][1]. In the Seasonal Adjustment Methodology section, it describes how X-12 ARIMA does it, though they also use a seasonal factor which perhaps you could also use or perhaps you could simply ignore.f


  [1]: http://www.sbp.org.pk/departments/stats/sam.pdf