It is easy to julienne (to make matchstick strips) once you know how to master the technique of 'squaring up'. Apply this way of cutting to any vegetable (e.g. leeks, carrots, tomatoes, celery etc) and you can't go wrong.
In the case of julienne of carrot: peel the carrot and divide it into 3 pieces. Take one of those pieces and square off the sides, cut the now square piece of carrot into 1/8 inch (0.3cm) thick slices.
Stack these slices on top of each other and cut into 1/8 inch (0.3cm) strips. Repeat with the rest of the carrot. For leeks, cut the whole leek into about 3 pieces then cut each piece in half and take apart the folds, stack them and slice as above. Go one step further to achieve the perfect dice. For tomatoes: skin, deseed, cut tomato into four, square up pieces all the same size and slice. Once you are at the stage to julienne as in the above carrot pictures choose how thick you would like them - the thing about this technique is having control. It will come naturally to you eventually and all your slicing will be the same length and thickness and will look very professional - as will any diced vegetable if you take that extra step. Use up all the left over pieces, after squaring up, in a salad or stock or casserole - don't throw them away!