Lauren hit the nail on the head. You have to trick your muscles with different movements to keep them guessing as to what your doing. Your body is a machine and will adapt to just about anything. Big time soreness that lasts longer than normal is usually due to working out muscles you don't normally use and the opposite of that is doing a workout and feeling good but not sore. It just means that you use those muscles a lot and it will take more to get them to that point. That is when you may want to push a little harder. Mix it up and you will be fine. Just remember to give yourself a couple rest days or move to a different part so that you have time to heal and then hit it all over again!
You won't always be sore. Depending on how good your nutrition is and your bodies ability to recover and how hard you really push yourself some people may never get sore. If you want to push yourself to the limits and are activating the right muscles rather than just going through the motions then there should be some degree of soreness. Need more info on how to switch on the muscles during certain exercises then feel free to ask.