This is a tool which I found very useful for Daniel. We did two small postcards. Each had a drawing of a straight line and a bumpy line. Daniel took the initiative and stuck little car stickers on top of these lines. Whenever, he stammered, we would show him the card and pointed out gently that he was speaking like a bumpy road. Then, when he spoke without any stammer, we would praise him and congratulate him for speaking like he is driving on a straight road.
Daniel was not upset when we used this method. As a matter of fact, he loved it and took pride each time he was shown the straight road card. He even made a few more similar cards himself. At times, when I was tired and hesitated a little when I spoke, he would gleefully say, "Mummy, you speak bumpy road!" and showed me the card.
You can also use visual aids by writing words which your child finds difficulty in pronouncing or short sentences on flash cards and teach your child the right way to say them.