Most of the tutorials I've seen recommends securing the string of lights every 12 inches with plastic clips. This should keep your lights straight, but if you notice that your string is drooping in some places, then try adding additional plastic clips to keep it in line. Good luck!
Are you using hooks made for string lights? Follow the roof line. Maybe they need to be placed closer together when you install them.
C Tiller, You need to use hooks that hang from your gutters or a hook with a screw, that you screw into wood. You can leave the screws in after the holidays. Aloha
Place your light clips closer together!
With the older C7 or C9 incandescent bulbs, this was easier, as the strings had straight wires and there was a clip on the bulb socket. Many of the newer LED styles have twisted, or linked double wires, and most I have seen do not have clips. If you like to have all your bulbs facing the same direction, it is next to impossible with some of these 'fancier' styles.
Is the frame of your house wood? If so, you might want to try these nail in clips. As they do require some patience to install, they are meant to be left up between seasons, so that next year, all you do is push the wires of the strings back under the clip. These come in different sizes and colours.