A couple of additional tips:

When driving on roads that are wet to the point where you can see the squeegied tracks of the preceeding cars, drive so that your tires are following in the tracks of the preceeding traffic. This can reduce a lot of the MPG loss associated with driving on wet roads. Also when driving in very heavy rain with LOTS of water running off the road, drop your speed down as low as traffic will allow. In addition to hydroplaning issues, an inch of water on the road can play havoc with your gas mileage and reducing your speed from 60 down to 30 MPH can greatly help reduce the MPG hit (think of the mileage that boats get).

If you can choose what time of day you can make your trip, try to plan it for the warmest part of the day in the winter or when temps will be in the 70F to 80F range in the summer. Also if you can, time your trip to avoid rainy weather and wet roads.