Abs are normal muscles. So you should work them like normal muscles. This means using progressively more and more weight just as you would your arms, chest, back, or legs. Do not simply resolve to doing more and more crunches or low (or zero) weight exercises. If you use the ab machine, make sure to go all the way up (so your abs are fully stretched out), but do not let the plates touch that split-second rest is enough to discount your set.
Don't worry too much about your abs "getting too big". That is very hard to do as a natural athlete. And, think about how hot a guy is whose abs ripple under his shirt! They will only do that if they are big enough.
For those few lucky guys who can get their bodyfat low enough to see their abs, one of the scariest things is to see a guy who has little fat, great arms/back/chest but no abs. Don't be one of those guys. Add weight to your ab program. Guys will drool.
For the rest of us non-ripped guys, the larger your ab muscles are, the easier they are to see under an extra layer of flesh. Remember that.
Shoulders are another neglected body part. You can dramatically change your appearance just with shoulder work alone. Shoulders are one of the most appearance modifying muscles, and seen the most second only to your neck.
Simple flies to the side are all that's needed to increase width. You can, of course, get more complicated. You can even do a few sets of side laterals almost every day to get your base down. I stress side laterals because your front and rear delts should get plenty of work on your chest and back days.
Another trick for shoulders is when doing cardio, skip the treadmill/bike on shoulder days and instead use the handbike or row machine. When using the handbike, push it's handles up as high as they can get and keep your elbows up and out. Do this for 10 minutes and your shoulders will burn like never before. That burn means your shoulders endurance and bloodflow capability is increasing -- and thereby your ability to work them with more weight later on.