Simple Linear Regression - Many Data Sets

We have to predict a future value for one variable based solely on past instances of that variable. This has to be done for many different data sets, and different formulas can be used for different data sets. We've been using Excel Trend and Average. For example, taking the most recent 8 data points, dropping the min and max values, and trending the remaining 6 points has worked well for some data sets.

Any general advice or suggestions for what we might try?

Thanks very much!