Some sweet potatoes are stringier in texture than others. I find roasting them yields the best results. But alas, if you find your sweet potatoes on the stringy side, use a Hand Mixer at low speed. The strings will naturally wrap around the beaters. Every so often, stop and clean the strings off the beaters, then continue mixing. You can also use a Fine Mesh Sieve. After mashing, press the sweet potatoes through a fine mesh sieve or strainer using a spatula or spoon. This will separate the strings from the mashed sweet potatoes, giving you a smoother texture.