In reply to Denni:
Hi Denni
Some good advice above. I'd suggest (in order):
1) Slim down your files. Use something like Daisy Disk or Space Gremlin and clear out the rubbish. That should help for a while but ultimately you need to do something else as well.
2) Buy an external HDD (no need for SSD - you have one already in your MBA and that's enough). On an MBA you can either use a USB (cheapest) or Thunderbolt (most expensive) without the need for any adapters. If you get a BUS powered drive it will be portable and not need an external power supply brick, which makes it much easier to carry around with you. They contain laptop 'portable' drives (2.5' rather than fat 3.25' desktop hard drives that need more power).
Cheaper large drives will need their own power supply and are best left on the desk.
3) Alternatively, assuming you have a MBA with an SD card slot e.g. for cameras, there are a number of companies who make cut down SD cards that slot in and add storage e.g.
http://www.amazon.co.uk/PNY-StorEDGE-Expansion-MacBook-Display/dp/B00DC9U2Q...
These slot in and fit near flush to the edge of the MBA - up to 128Gb of solid state storage - no leads required. I have one in as I type this and it has worked perfectly so far. I also have a LaCie Thunderbolt rubberised 1Tb portable drive which was more expensive but also much larger...
4) Lastly somewhere like OWC can sell you replacement SSDs for MBAs - big 480Gb drives and a screwdriver are about USD400 if you are feeling brave and want to crack open your MBA...
If you aren't sure what model of MBA you have then everymac.com can tell you.
HTH
b