fixed interest rate

I am a first home buyer. How do I determine whether I should go for a fixed or variable rate? What are the things to remember? Also, is it a common practice to negotiate a rate (either fixed or variable) with the bank? If yes, can I still negotiate a rate after I have been pre-approved?

The recent ABS Housing Finance Commitments shows the proportion of fixed rate loans taken out for new mortgages and refinances for the last year. From Table 9 on p17. In March 08 ~24% of new loans were fixed rate, in March 09 only 2.4% were fixed. Why do such a large proportion of...