Here are a few simple tricks for car-camping in the rain. Most of these may be well-known to people who camp in the wet... but for those new to it, here are some little lifehacks we've put to use in various state parks here in the northwest.
Step 1: Tip 1: Raise that tarp
When car-camping in the Pacific northwest, the first order of business is often to get the tarp in place.
To start, tie a rope around a tree near your eating area. A good knot to use is a bowline knot ("the rabbit comes out of the hole..."). You can also use a "no-knot," in which you simply wrap the rope a few times around the tree; friction will keep the rope in place.
Keep the tarp high enough to allow good light in. Use a forked stick, or a "chuck-it" ball-thrower if you have one, to coax that rope higher up the tree.
Step 2: Tip 2: Forget the grommets
Don't bother with those little corner holes in your tarp; they're not strong enough to handle the tension you'll need for a good rain-shedding cover.
Instead, take a tennis ball or a similar-sized rock and wrap it into the corner of the tarp. Tie off with a few wraps of the rope and some half-hitches.
Step 3: Tip 3: Rig a drying line under the tarp
If you have a long enough rope, run it diagonally across the tarp and tie it to another tennis ball in the opposite corner. This helps prevent tarp sag, and can also serve as a drying line for light items.
Step 4: Tip 4: Cinch up the tarp tightly using a trucker's hitch
You need to stretch the tarp tautly across the diagonal so that it sheds rain. A loose tarp will flap in the wind and collect puddles of water overhead.
To cinch the tarp tightly, wrap the rope around a tree and bring it back through a slip knot tied into the main line of the rope (the part running diagonally from the tennis ball). Pull the rope back from the slip knot. This technique, called a 'trucker's hitch,' almost doubles your leverage for pulling the tarp tight. Tie off the tightened rope with half-hitches.