Wear bright colors for visibility, do not spread out across the road, wear sunscreen, bring some over-the-counter pain relievers (seriously), do not feed the cougars, and make sure you have a chain tool in your toolkit (and know how to use it). If you come to a long steady uphill, you can shift to a high (slow-turning) gear and stand up, and just sort of do a steady Stairmaster approach to give your back and bottom a rest for a minute or two.
Clothing-wise, what I like is to wear a soft wool dress sweater against my skin (local thrift store is great for this), then put a large T-shirt over it. If it rains, the wool doesn't like to wick water from the cotton, and it's warm too, yet breathable. A windbreaker is also good if you hit serious rain, but you need insulation under it. Which I found out the hard way once, doing a 75-mile ride from Pullman to Spokane in... February? I forget the month, but I sure remember the lesson :Q Used to ride that route a lot, sometimes both ways in one day.
If you want to pick up a good windbreaker, REI's neon-yellow
Rainshadow jacket might be worth a look. Not too expensive, very visible color, it's rainproof, and has big underarm zippers for when you want some airflow. I wear one of these for commuting visibility.