Any ideas on tightening outdoor volleyball nets?

Hey guys, there are a lot of public outdoor sand and grass nets in my area. Most of them are on a pole system such as this: http://www.c2cturf.com/images/projects/volleyball/001.jpg

Unfortunately over time these nets always start to slack and be too low. Are there any crafty volleyball players out there who have figured out a way to raise them? Maybe with carabiners or more rope? Thanks!