Having programmed a lot is a big advantage, but you're going to be learning a wide variety of things as a CS major that you may not have had to think about previously, and there is going to be a lot of it which isn't intuitive. Be prepared to study more than you have in the past and don't get cocky; sometimes it isn't until your junior or senior year that you get beyond what you already sort-of know and hit a wall.
2. Try to double major or minor in something completely unrelated to CS and math, unless you are really into CS or math.
Programming is a tool to make computers do stuff. A pure CS degree leaves you qualified to program compilers and IR systems and computer games. If you learn about something really hard, like chemistry or biology or whathaveyou, you'll also know what sort of programs chemists and biologists and whathaveyou need and have a domain to write programs in.