The key to engaging your abdominals during deadlifts is to keep them braced into your back (your abs should remain pulled in, but you should still be able to breathe comfortably) during the entire movement. If you do this, your obliques kick in to prevent shifting from side to side and your abdominal wall helps support and stabilize your spine.
How to do it: Stand tall with your feet hip width, holding heavy dumbbells or a weighted barbell, palms facing in. Pull your belly button in toward your spine, soften knees, and keep spine naturally straight as you hinge forward from the hips until your chest is almost parallel to the floor, arms reaching down in front of legs. Maintaining a straight spine, slowly return to start. That's one rep. Do up to 3 sets of 12-15 repetitions.
