I used to take sim cards to my local petrol station, local corner shop and charge 50p (shop kept the 50p).
That stopped though when giffgaff started supplying their own sim cards to the same shops. So I've stopped doing it that way.
I also find if you give something to someone for free then they're less likely to bother with it as it hasn't cost them anything.
I was at a car boot sale last year and plenty of people took a sim card (free of charge), not one was activated.