My father used to have a flat elevator weight he would bolt underneath the bed on his 1/2 pickups for ride quality. That's also accomplished with the Toyota acessory called "ShurTax All Weather Traction". It's an easy way to add weight in the bed and is basically a big water bag:
Toyota Tundra - Truck Accessories
If it were me I'd probably just throw some sandbags in the back (like Art64 did). If you drive around unloaded all the time just leave them back there.
And it does in fact take a little time to break in the suspension on a new vehicle. Instead of loading the sand bags you could also go on one good trailer tow trip (i.e. 4,000lb loaded bumper pull trailer).