And of course the best way to encourage your chidren to do anything is to lead by example.
Imo a lot of the "I spend all day and night on the PC" talk is because the parents are just sat watching tv or whatever themselves.
I make a point of taking my little boy to the park to play football etc. at least 4-5 times a week. Hardly ever is there anyone else there, apart from the usual selection of goths skateboarding.
And we both benefit from the excercise.
