Befor we left the Uk 2 years ago, we contacted some agents and asked for their advice; some were good, others just bloody rude!
However, we did get some good advice from one guy; bring with you some references ( we got 1 from our vicar neighbour, my boss, and our bank manager!); enough money to pay a months rental in advance (no mean feat with a family of 6 to house!) and make sure you contact them as soon as you can when you get here!
We were lucky, a big persistant family gets remembered!!
Also, dont limit yourself to being very central; as said, you can travel fairly easily around the metro area...........anywhere within 10 klms, but you will pay a premium for a place near the CBD.
Keep at it, and good luck