Supposedly the safest way (according to banks) is to sign on the back of your card as soon as you receive it. So that when you shop, your signature be matched to ensure you are the one who has authority to use that card.
But in my personal opinion, it is very risky to sign on the back of your credit/debit card. Because by signing on the back you are providing a copy of your signature so that any ill-willed can try to make a copy of that signature and pretend to be you. At the same time if you don’t sign, then also you are at risk. As then you are opening more door to fraud.
Instead what I generally do on my credit cards, I write ‘See picture I.D.’ to make sure that the merchant always sees my drivers license or state ID before approving the purchase. So I am saved….Of course the transaction sometimes takes long time but it is always safer in my opinion to do so. At least I am not paying any hefty bills on my account.
What do you do on the cards? Do you sign?