Besides making sure you are not in a low lying area, if the ground you pitch the tent on is at all sloped (even slightly), take a stick and drag it in the ground to carve a shallow trench (couple inches) just above the tent and just around the ends or sides of the tent. I have a huge amount of experience backpacking/backcountry skiing/camping in all kinds of weather, and this trick helps to guide any run off around the tent. Carve the trench just barely away from the tent: just under the rain fly edge. I have been doing it for the last 40 years if it looks like it might rain or if it is raining. Many of the other suggestions are good ones. You do not have to be miserable just because it is raining and you are living in a tent for the moment. I always think it is kind of fun and cool to be so close to nature.