Best way to mount a slider?

So, I'm about to put a slider in the back of the cove where I live, and I need a suggestion on how to hold it down. THe ground is pretty soft back there, with the ocassional rock, but it's mostly clayish mud. What would be the best way to hold the 4x4's down? Driving them into the ground or making a kind of platform under each one and weighting it down with sandbags?

Drive em into the ground then take long survey type stakes and drive those in right next to them a ways down like almost to water level then screw those into the 4x4s thats how i make em in FL and all we have is sand lol