The agent is supposed to work for both because he/she gets the commissions from both sides. In reality, they would side to the tenants because they are in a quick deal. It is far better to get a deal than to get a high-rent deal because the extra time is just not worth the extra money. In this sense, they you can tell the agent FIRMLY (non negotiable kind of bottomline) the bottomline and he/she will work on that figure for you. The longer he/she spend on it, the more eager he/she wants to close the deal or he/she would waste the time already spent. 10% off the advertised figure is a good start, give or take another 5% depending on your bargining power (as suggested by traffic), the landlord's experience of non-Chinese tenants etc and the market (aparently there are fewer flats to let now because everyone want to sell to capitalise the rising property market - not sure if it is true).
All the best for your hunting.