I really struggle to get much return on cash. If you have a kid it is a bit easier as there are some decent savings accounts out there paying 3%+. My basic rule is that I save whatever I can at rates that are higher than my mortgage interest rate; and use the surplus to overpay the mortgage.

My current plan for monthly savings is as follows:

My firstdirect reg saver (5%): £300
Wife firstdirect reg saver (5%): £300
My HSBC prem reg saver (5%): £250
Wife HSBC reg saver (5%): £250
Santander reg saver (3%, but now 2.5%): £200
Halifax junior saver (4.5%): £100
Barclays junior saver (3.5%): £100

The harder part is working out what to do with the money when the 12 months is up. Currently we stick the cash into nationwide accounts paying 5%; or the nationwide junior savings account paying 3%; or use the money to do stuff on the house. All of the above returns more than my mortgage interest rate, and is effectively a DIY offset giving me flexibility.