Prepare a rainy day time line considering extra time for travel.
Contact your vendors to see how rain could affect their service.
Caterers can help by altering the menu to offset cold weather discomforts.
Know where, and how much notice is required, to obtain outdoor tents.
Your wedding venue may have this option available.
On a smaller scale have a supply of plastic tarps that can be used to quickly cover chairs, tables, etc..
Consider event insurance. This would only be used if the event was canceled due to a serious weather event but can protect your investment.
Plan for make up and hair styles that can handle rain. Arrange with your stylist, that in the event of a rainy day, to come to the venue prior to the reception.
Work with your photographer.
Plan in times that you could step outside with the photographer in between showers.
Worst case you will want a "back-up" indoor location.
Have a plan to prevent mud on outdoor paths. Options include ground cover (i.e. ,wood chips, hay)
Wet Floors can be maintained by having a stockpile of large beach towels.
Prepare for the guests needs.
Make sure your guests know the event is outside so they can plan appropriately for their family.
Provide coordinated colorful umbrellas.
These can even be embroidered and given as parting gifts.
Matching set of rain boots and umbrellas for the wedding party.
Hair dryers, towels, and toiletries in the restrooms