There are limited methods of increasing the intensity of ab exercises, especially by adding weight.  Generally, intensity is increased by performing more reps or working them for a longer duration and the abs generally respond very well to that stimulus.  However, sometimes it is nice to just add some weight and perform the same number of reps.  Cable crunches are one exercise that allows you to do that.

Starting Position

Position a cable machine so that it has an overhead pulley and set your desired weight.  Attach a rope attachment to it and grasp either end of the rope with a hammer type grip.  Kneel on the floor so that your arms are slightly out in front of you while holding onto the ropes.  Extend fully and then bring your arms down so that your forearms are tucked against your ears.  You should be feeling your abs slightly stretched at this point.

Performing the Exercise

Using your abs, crunch, curl downward rounding your back pushing your head towards the floor.  At the bottom when you are fully contracted, hold for a second and then slowly release back to the starting position.  Continue for the desired number of reps.

Tips

The biggest concern here is to ensure your arms are not doing any of the work.  Keep them locked against your ears and do not pull down with them.  As well, ensure you are back far enough from the machine to maintain constant tension throughout the full range of motion.  If you are too close, you will lose some of the benefit on your lower abs.  There will be a maximum limit to the amount of weight you can do this with without anchoring your legs to the floor, simply because your bodyweight will not suffice.  If your abs are getting that strong, good, and simply have someone/something anchor your ankles/legs done.  However, doing so will mean your legs will take some of the work away from your abs.  It's a tradeoff.