Rachel - that plank thing, we call it a GI Jane!! I hate that excercise!!!! You can do it on a excercise ball too, that is excellent for those flabby abs.
You can even do it the wrong way around. Sit straight up, knies bent slightly but comfy away from your body, lean back and put your hands - fingers towards your butt - on the floor about 20cm from your butt. Breath in and push up, keep your body straight, head back and your abs in, keep position for 20 counts, do three reps. Great for back muscles and that stubborn shoulder muscles - you'll feel what muscles I mean
My favorite abs buster is the vanilla plain crunch. I LOVE doing crunches and crunches with your hands straight up in the air and reaching for the roof. For obliques I like the one you take weights and keep them at your sides and just bend from side to side and CRUNCH as you do that. If you really, really consentrate hard on the obliques, you get excellent results. To give a whole new meaning to pain just lean ever so slightly backwards and CRUNCH *ouch*