Here's a few things you can try
I’ve ran across a few people who have upgraded their Macs to Lion and are having trouble emptying the trash! Apparently, it gets stuck and the progress bar never finishes. If you’re having a similar problem with your Mac, then try out a few of the methods below to fix the issue.
The one solution that seems to fix the problem is to open Terminal and type in the following command:
This will basically delete the trash folder on your Mac, which will force OS X to rebuild it. This usually fixes whatever problem there may have been with the original trash folder. Note that you will have to type in your administrator password and it will be blank while typing (you won’t see the letters).
Another quick thing you can try is to press and hold the Option key and then clicking Empty Trash. This usually will help trash any problem files that you may have.
Securely Delete Trash
If you still cannot empty the trash, you can try to securely delete the trash. You can do this by going to Finder and then clicking on Secure Empty Trash.
Sometimes the trash will not empty if there are files in the trash that are currently in use by a program and therefore are locked. This is a bit tricky because if you have thousands of files and there are only a few that are locked, then it’s hard to find them.
You can try restarting and see if that help. If not, you can download a program that will delete all the unlocked files.
Once you delete all of them, you’ll be left with locked files. Now right-click on each file in the trash and click Get Info. In the bottom left corner, you’ll see an option to unlock the file. It’s a check box like below:
You should then be able to delete the locked files. Unfortunately, OS X does not delete all unlocked files and leave the other ones. Instead it just deletes the files in order and if it runs into a locked file, then it just stops! Annoying. Maybe they’ll fix that later.
If you’re still having problems emptying the trash, post a comment here and let us know what you tried and I’ll try to help! Enjoy!