How can I automatically have my Trash emptied when disk space gets below a certain point?
I use a third party preference pane Compost that deletes from trash when the disk fills up or if the file has been in trash for a given time. 

The original download stopped working for me but I have just found this on the author's site instead of the distributer. Note the author's comment


  Compost is in a bit of a state of flux right now. The distributor has moved on to a different market and we wish them well in their endeavors. Additionally, the changes necessary to fully support Snow Leopard have been going through an extensive beta period as a result of the degree and complexity of changes required. Please stay tuned for more information.


So it might not work even from this download

When I updated this I had just installed it on Yosemite and it installed (but not enough time to see if it deleted anything) I have now updated to El Capitan and the installer fails.