I have used my leveling boards as a base for a jack. Also I have large wood blocks, 4x4 up to 8x8, that can act as a spacer if the jack doesn't go high enough. Sometimes just a couple short pieces of 2x4 will do the job.
I have also used "jackstands" when needed - lift up the trailer, put jackstands in appropriate places to hold it up, reset the jack and maybe add a wood block to lift the trailer higher. Wood blocks can take the place of jackstands - depending on the situation, sometimes they work better.
Usual warnings apply about working on soft ground with improvised materials.