The most obvious solution is to sell them. You would be surprised at how many people are willing to purchase used solar panels for the simple reason that they are far cheaper. Investing in a brand new solar panel system isn't cheap and is beyond the means of many people. And yet many people still want to do their part and be as eco-friendly as they can be. Purchasing used panels to provide solar power is one option available to them.
Provided your old solar panels aren't damaged and still work, selling them is an attractive option - it's a smart buy for the buyer and a great deal for you. And the money can go towards newer and more efficient panels. The old panels go to a new home and don't end up in the dump.
If you are going to sell your old panels, make sure they work well and you can show the buyer how they work.
If you are feeling more generous, another thing you can do with your old solar panels is give them to a friend or family member, or 'freecycle' them to someone in your community. Many people are reluctant to make the large initial investment in a solar energy system - but if the panels are given to them, the initial outlay isn't quite so steep and they have the opportunity to help the environment by moving away from the use of conventional fuels.
You may also want to see if universities or research facilities are willing to take your old solar panels for research.
Of course, if your solar panel is no longer working or has been damaged, there may be no recourse but to throw it away. Perhaps in the future, recyclable solar panels will become a reality.