How to Use iTunes Match With Very Large iTunes Libraries
iTunes has the ability to access multiple music libraries. By separating out the songs to sync with the songs not to sync, it is possible to force iTunes under iTunes Match's 25,000 song limit.
To do that, quit iTunes, hold down the Option key, and launch iTunes. You’ll be prompted to create a new library or choose a different library. Choose the option to create a new library. iTunes will open and you’ll have nary a tune in your library.Rather than steal all their thunder, head to Macworld for the full instructions.
Move to the Store menu and choose Turn on iTunes Match. You’ll be prompted for your Apple ID and password. Enter them and click OK and iTunes will switch on iTunes Match. Now open iTunes' preferences, click the Advanced preference, and uncheck the Copy Files to iTunes Media Folder When Adding to Library option and click OK. This will prevent iTunes from generating duplicates when you follow the next step.