About a ground sheet under the tent: If your tent already has a waterproof tub floor then I advise against using a ground sheet. The ground sheet may just trap water. If you must use a ground sheet under your tent then make sure the edges don't protrude beyond the footprint of the tent so that rain doesn't land on it and flow under the tent floor.

You want to also pitch your tent on ground where the water will flow away from it and not form a puddle, perhaps even at the expense of sleeping on a slight incline. That clear, flat, smooth, level piece of dirt may look inviting but that can be an indicator odwhere water will pool during a downpour. Look for water flow marks on the ground to get an idea of where water will flow during a storm. I have found that, given a reasonable (but not heroic) sleeping pad (in my case an inflatable thermarest) that pitching and sleeping on gravel or hand-sized stones is surprisingly comfortable and allows good drainage even though most peoples' instincts are just the opposite.

Also, keep in mind that high winds can bring down trees so look upwards to judge whether there are rotten limbs ready to ruin your day (or possibly life). This actually happened to a small group I was with. Fortunately, we had just broken camp and were standing around ready to leave when a tree fell where some had been sleeping.

Pitching a tent in the rain is tricky. You want to be as fast as possible so practice is a good thing. Some mods to the snaps or clips might be in order so they work easily and quickly when wet or slippery. With some tents is it possible to pitch the fly first and then slide the inner tent in underneath so that it is protected while you get it up.

Pack your bags in a manner so that you can get the tent out first leaving your clothes and sleeping bag safely protected.

Hope this helps...Or maybe I'm just all wet.