I'll tell you what I do.  I've got 4, and am still working on the bottom two.  
I lay flat on my back, with my hands behind my head, and my feet flat on the ground, legs bent.  I then do a sit up half way, and bring my pelvis up towards my chest (just in that direction).  That will be a nice crunch type excercise.  Note that your feet should not be pushing your pelvis.  In fact your feet and legs should be motionless, they aren;t participating in this excercise.  Use your abs to move your pelvis up.  Do about 30 of those, and never release the tension once you've started (i.e. never go back down so that the muscles relax.  keep it tight).
Also, I do an alternative to work on my obliques.  I generally do this at the gym (which I go to 5 days a week), on the decline bench.  Secure your legs, and put your hands behind your back.  then do sit ups, except twist your body all the way to the left while you are comming up.  Do that about 10 times per side, again making sure not to release tension on your ab muscles.  So for the other side, turn your body all the way to the right while comming up.  Don;t give yourself a hernia, because you CAN do that while attempting these excercises.  But hey, they;ve worked, and continue to work for me.  Then again, I also have genetics on my side