Everyones circumstances and travel patterns are different so you will need to do some research as to what options will work best for you. However at minimum you should be getting 0.58p/avios and a good use should be anywhere from 1p/avios upwards.
See also:
http://www.flyertalk.com/forum/briti...7-edition.html
http://www.flyertalk.com/forum/briti...tart-here.html