What level of Medallion are you? My suggestion would be to pay close attention to your seat assignment around your C+ upgrade window, as you can immediately change into a different/better seat as soon as you're upgraded. If you're an FO, or even a Gold, there may not be many other seats left at that point, unfortunately.

That said, I'd still watch the seat map up until boarding, as some of the better C+ seats will open when higher Medallions get upgraded into FC. I've improved my seat many times right before boarding using this method. You pretty much have to watch the seat map religiously, though.