The best way IMO would be to get an external hard drive or USB stick (or you could use DVDs and CDs). If you use a hard drive, you could copy the files across, put them in the same place on the MBA, then simply start iTunes while holding the "alt" key, then navigate to the location of the library.
For the CD/DVD method, you could do exactly the same thing, just that you would have to split the music folder on to different disks (just split it by artists for example, or one disk from A-H, then I-Z or whatever), then reconstuct it again on the new machine.