The exercise where you lie flat in your back and then raise your legs straight up (so you are at a right angle) and then lower slowly just above the floor and raise again, lower raise lower raise (you get my drift) is really good.
Make sure you keep your arms by your sides and control the movement. It does hurt after a while but pain is gain!
Another good lower core one is to lie on your back with your legs up and your knees bent so your lower legs are parallel with the floor (the "table top position") - then slowly lower each leg in turn and tap your heel on the floor.
I got a fabulous DVD called 'Blast Off Belly Fat' -it's from the 10 Minute Solutions range. Even when I was running 3 miles 3 times a week, my gut didn't shift but this has made my abs so much stronger and flatter. There 5 different workouts, each are 10 minutes long, you do as many or as few as you want. I do 10-20 minutes about 4 times a week. I could notice a difference after about a week. Just do as much as you can.
I used to struggle to hold a plank for 3 seconds, now I'm working towards 60 seconds! They really work.