Macbook Air hard drive - nearly full warning
I would recommend looking at using a utility like CleanMyMac 3 to go through and perform some system maintenance and free up extra disk space. The first time I ran it on my system I freed up about 20GB of hard drive space by getting rid of language packs that weren’t needed by my system. It will also go through and clean out duplicate and unneeded version of photos from your iPhoto library.

There are several other apps around that can do the same thing (MacCleanse, Dr. Cleaner), but I’ve found CleanMyMac to be the best of the bunch.

Also, make sure to reboot your system at least once a week. This will let the Mac OS do it’s own system cleanup that doesn’t happen until you reboot your computer.