Standing abdominal crunches.

• Front ab crunches
  
  
  
  Stand with one foot forward. With the leg bent, raise the leg as if you were trying to put your thigh on your chest. Take care to rotate the hips so that your can point your navel at the ceiling and feel a good pinch in the abdominals. Height is unimportant, just be sure the feel the crunch.
  
  
• Oblique ab crunches
  
  
  
  Stand with your toes both pointing straight ahead. With the leg bent, raise one leg exactly to the side. You will need to let your butt go back some, which will cause the leg to be at 45 degrees. This works the obliques nicely.
  
  
• Side ab crunches
  
  
  
  Stand so that your feet form an 'L'. The bottom of the 'L' is the leg that will go up. Roll over a bit so that you are slightly facing the floor as you raise the bent leg. If you know what a high side kick is, that is essentially your final position. You should try to flex the gluteus medius at the end of this crunch. Strengthening this muscle will help you stabilze in walking and help you avoid knee pain when doing pistols.
  
  If you have trouble finding this muscle, then lie on the floor on your side with your legs extended and lift your leg straight up (this is the final position of the standing version). You should feel the muscle, which is on the corner of your buttocks activate.
  Rear ab crunch
  
  
  
  Stand with your feet together and toes facing ahead. Shift your weight to one side and lift the other leg, keeping it slightly bent. Make sure you feel the pinch in your lower back. You can't get as high with this as the others.
  
  
  How to work up to it: Situps are the best way to work up to these.
  
  Ramping it up: Pinch harder. There is not much of an upper limit to that, so do it as hard as you like.
  
  Do's and don'ts: Don't have the leg straight, since this makes flexibility come into play. These are actually a really good thing to do early in your workout as a warmup.
  
  Comments: These sure look like leg lifts, don't they? They are most emphatically not though. You must 'pinch' yourself at the waist for each if these. If you don't feel the actual contraction you are just swinging your leg. You could easily replace all your situps with these. An interesting aside is that people who kick in martial arts spend a lot of their conditioning time trying to get the hip muscles to hold up the leg. This is not such a good idea since it is the abdominals that actually provide the stability. Weirdly enough, even though I don't kick, I can usually get my foot higher than people who do just because I get good rotation of the hip and do a good ab crunch.