Rep,
The following is advice a mate of mine from the ATO gave me recently:
A couple of weeks back when I was around your place you mentioned a question about decline in value on something you purchased for work. I'm not sure how much you already know about claiming the deduction but the attached link provides the basics for how you would calculate a deduction.
http://www.ato.gov.au/print.asp?doc=/content/35040.htm
Calculating decline in value involves identifying the effective life of the asset. The Commissioner has determined the effective life of some assets which are set out in the Ruling referred to on the page above. However, I'm not sure if the asset you are using is included. If not, you can self-assess how long it is likely to last. Relevant factors to consider are also included in the Ruling.
My self-assessment of the useful life of my headset was 10 years. I'm sure it will last longer though.
Hopefully this helps.
Ando