[nuccy]

Basically two methods:

1) have counterweights (e.g. as mentioned above for another similar concept Cargolifter which used water);

2) expand its volume during loading and contract after unloading. E.g. pumps compress gas while unloading and release it back while loading.

These two options are obviously the same thing - change the vehicle density to adjust the buoyancy but implemented with and without external weights. Submarines do this also by filling the ballasts tanks with either water or flushing them with compressed air.