The natural rate is equal to the structural rate plus the frictional rate. Neither of these two will be reduced in the long run by increasing aggregate demand, or so goes the theory anyway (and despite my opposition to it, it is quite accurate).
So in order to reduce the natural rate you can reduce:
1. Frictional unemployment - reduce the amount of time workers spend looking for a new job. The government did some reforms in the mid to late 90's to do this (replacing CES with Centrelink and a "privatised" job network) though I'm not sure on its effects. Internet job ads have helped to reduce it.
2. Structural unemployment - reduce the number of people who are not qualified for the jobs wanted by employers (so that if you increase AD there will not be any qualified job seekers to work in the job vacancies created). This is done through retraining schemes, higher funding for higer education, encouraging teenagers to stay on till the end of year 12 (plus go on to TAFE/uni/learn a trade if possible) and assisting wokers in gaining additional skills throughout their working life.
So in theory it is possible to have a natural rate that is very low as in the 50's and 60's when Australia had an unemployment rate of about 2%. Some think that the baby boomers reaching working age following this time was the reason for Australia having an unemployment rate of 5-10% over the 70's, 80's and 90's and that once they start exiting the workforce Australia will return to low unemployment rates. Which means that the biggest labour problem in the near future will be a shortages of labour, not a surplus labour.
For many of you, it means easy jobs!
For others, it means that your business will have trouble attracting and keeping good workers.