I noticed the Odyssey used to sag when it was loaded up with heavy cargo. I was able to fix this by adding Air Lift suspensions to the rear of the van. Air Lifts are just these red rubber cylinders place in between the rear springs. When the van starts to sag all I have to do is just inflate the cylinders and the van would slowly start to raise. It comes really close to stock height.
The instructions it comes with are very detailed. It has pictures taken from an actually Odyssey and shows you what and where to do things. The installation itself is not the easy thing in the world - but it's not impossible either. The whole thing should take about 1-2 days.
Click on image to enlarge
Here's the actual instructions. The only page missing is the circular template that helps you know where to drill under the control arm. Click HERE for the Adobe Acrobat file. The file size is about 3 MB.
I purchased my kit at www.carparts.com. Currently it's $80.00 for the Air Lift 1000 kit (part# 60740). Keep in mind - this is the basic kit. There's other kits that provide the Air Lifts and an on-board compressor. For more information on Air Lifts go to: http://airliftcompany.com
If you have any questions email me at email@example.com. Good Luck!