The most important thing you can do is to stay dry. Once you get wet you will probably stay wet especially with the perfect sailing wind you will feel chilled and not ever get warm. Saltwater is incredibly hard to dry because of the salt crystals. If you sail because you love it, your fashion sense won't matter and you will do whatever you need to stay dry. Waterproof clothing and shoes are great but if you don't have proper wet weather sailing gear you can always layer warm clothes underneath. If you find yourself in a squall you do not have time to get waterproof clothing on so something like a simple plastic bag over your legs can help.
Layers can be perfect for changeable weather. You can pull them on or off easily when the temperature rises or drops. If you are on watch you won't want to disturb the other crewmembers were arrested so this will make sure you can be comfortable for the duration of your shift by having several options. Breathable clothes are great because they wick sweat away from your body.
Stay out of the breeze by staying behind the dodgers and utilize any coverage you have. If you're unwatched you get up every 5 to 7 minutes to look around and stay active by tweaking the sails or raising them and safely walking around the deck can keep you warmer.
Soggy wet salty cushions take a long time to dry and are not comfortable to sit on. Remember to take the cockpit cushions inside to keep them dry also.
If you stay warm you will have a great day sailing.