Excel - Tendency line as simil to x12?

Hi
I have some graphs and i would like to plot tendency lines for them.
I don't want to plot something more complex as a x12-arima series, but i'd enjoy a tendency line with more basis than a simple linear regression.
The data i'm plotting are monthly sales.
They suffer high seasonality and some items are high in winter and others high in summers.
Do you think a moving average of 6 periods is ok?
What do you suggest?
What parameters would you use in the tendency line options in excel?
Thanks for your comments and time.