I was introduced to rain-x a couple years ago and I've found it a fantastic car maintenance item. Rain-x is basically a wax that is perfectly clear. It protects your windshield and keeps the rain rolling right off.
Step 1: That Dirty Windshield
First, clean that windshield!!
I use Windex. It works well. Just spray it on and scrub off the grime with a paper towel. Dry with a paper towel.
Step 2: The Clay Bar
The clay bar is an extremely useful tool. It picks up embedded dirt and grime from the surface and traps it.
To use this - first wet the windshield lightly with some Windex. Then, slowly rub the clay bar over everything. Don't worry about the greasy lines that show up.
When complete, wipe down with a damp paper towel.
Step 3: Windex, Again
Cleanliness is key to the rain-x working properly.
Windex as before. Take care to ensure the entire windshield is dry before the next step.
Step 4: Add the Rain-X
Now, rain-x comes in several varieties. I always get the original in the yellow bottle. The other ones include cleaners, all in ones, etc. But I found that they don't stand up over time.
Spray the clean windshield with the Rain-x, much like the windex.
Use a clean paper towel and gently work the rain-x into the windshield.
Finally, get another clean paper towel and wipe off the excess. Take care that the paper towel stays dry. Any residue left causes streaking. When removing the excess, work in a circular motion.
Now you're all set for the next rainstorm!