If you use the video like x HeroBlade x said then you are quite safe, BUT sometimes it can happen that you step just slightly on the sand and no antlions will appear as an indicator but you won't get the achievement.
It is much easier to tell if you touched the sand if you are elevated (other indicators are shaking controller or shaking screen), therefore you should use two bed frames that you can jump on. Save after each jump and then take the frame from behind you and place it in front of you.
The two frames can be found here:
1. In the first barn after having used the long plank to cross sand for the first time (Video around 2:50). Shoot it as close to the last rock with a house on it (see 2) with the gravity gun.
2. Inside the last house on the last rock before entering the area with sand only (Video around 3:33).
There are actually three you can use, one's way down on the beach from one of the cliffs, but you can drag it up with the Gravity Gun. Makes it a bit quicker on the end stretch. (Thanks Hurball)