It's been raining a lot recently, and I have often been getting my pedals wet, they have been quite slippery when I get in the car.
Is there anything I can do to them to make them grip better in the rain?
Although your pedals may be beginning to wear out, I have found that even new pedals can sometimes be slippery depending on what type of shoe.
If you apply non-slip tape to your pedals, it will make them much less slippery. This tape is widely available, commonly used in RVs, and is basically adhesive 80-grit sandpaper.
Get some grip tape and put it on your pedals
Grip tape will allow you to use your pedals even if your shoes and wear we and give the traction you need to prevent your feet from slipping off of them when you use them.
Grip tape is used on the decks of watercraft and boats as well as the tops of skateboards to accomplish the same goals you have, more friction.
You can use a pair of scissors and apply a few strips of the tape to each pedal.
Grip tape is often used on pedals in racing vehicles to accomplish the same task.
You will find this to be VERY effective. I do this on all my vehicles and have been very satisfied with the results.
I'm going to throw this out there too as a supplement piece to the above answers oriented along the lines of preventing the pedals from getting wet in the first place. I however also recommend the grip tape solution as it will definitely maintain significant friction even when wet and will have a long lifespan as the sand granules slowly flake off and the adhesive slowly lose its grip beginning at the edges.
Consider replacing or installing some fabric/carpet floor mats that would ideally soak up enough water from the shoes to diminish the transfer of water from shoes to pedal. Also if they're already wet use the car vent button that directs airflow to the feet area to dry off the pedals and shoes during driving.
I found that putting cotton socks - yes gym socks - over my foot pedals and secure them in place with tight rubber band or string works perfectly!!!! Use black socks so they don't look took so dirty. Its a temporary fix of course - but it will get you to your destination and back safely, and can tide you over until you find your permanent solution.