How van you paint staraight lines when the texture of the walls is rough?

I'm painting my house and the corners where there are two colors together the line are nor straight.. Does not look very good Help.How van you paint staraight lines when the texture of the walls is rough?
I don't know exactly how rough the walls are but the first poster is right- painter's tape if they are only moderately rough. There is really no way to make totally straight lines if the walls are spackled...unless you sand them first...How van you paint staraight lines when the texture of the walls is rough?
Paint the lighter color on first, going around the corner. Using a very small brush, follow a pencil line mark; the darker color will be put on last. If you still have crooked spots, do touch up using whichever paint color will correct the problem; again, use a small brush and go slow... the final brush could even be a small artist's paint brush, not a brush meant for walls - it's the best way to do fine details like you need.
Use painters tape to make straight lines.