Put the two in the front on the rear area of the framehead near the pan, this is also sometimes called the "firewall" area just next the master cylinder. In the rear I put them on the torsion tubes.

Another thing I sometimes do, when space allows, is place a large cement block in the center of the car or near the area that I'm working as an extra precaution, or a good floor jack.

The best thing to do once you have the car up is to shake the car VERY hard and see if it moves or falls. Best it fall during a test and if it doesn't budge then you'll know it's secure.