It's important to keep water from pooling on a tarp because it can cause the material to become stretched out and damaged. There are a few different ways that you can do this. One way is to use grommets, which are metal eyelets that you can put in the corners or along the edges of the tarp. You can also use weights, such as sandbags, to hold down the corners of the tarp. Finally, you can use guy lines, which are cords that you tie between stakes in the ground and the corners or edges of the tarp. Whichever method you choose, make sure that your tarp is secure so that it doesn't blow away in windy weather.