How does one find a good letting agent?

So, the countdown has begun - we fly over to start our search for a new place on August 24th.  After a few days exploring London, we will be ready to tromp all over Kent, Surrey and Essex looking for a new home.  But how do I decide on a letting agent?  If I were here, I would go to yelp.com and look for recommendations, but is there something like that in the UK? 

Unfortunately, estate agents in general don't have the best reputations and they aren't registered or regulated although I believe there is some sort of voluntary association they can join.  They also tend to be very neighborhood specific so an estate agent in Kent is probably not going to be able to help you in Surrey.  When we decided on a neighborhood, we pretty much went to every estate agent who covered that area. 

So you have to decide on a neighborhood, then choose a letting agent (or a few) afterward?
I have been following a few towns on the rightmove.co.uk - would I contact one of them before I come over to make an appt?  Is there a fee for each agent you contact, or do you pay them once they find you find a place? 

Most estate agents won't talk to you until you're ready to start viewing properties.  You'll pay their fee when you sign a lease.  It'll be part of the deposit. 

You'll probably be better off figuring out which area you want to settle in and contacting agents to make an appoint to view a property.  I contacted the listing agent for a flat on findaproperty.com and when we viewed it, I wasn't interested.  He said he had something else that had just come available up in Finsbury Park, if I wanted to go that far out (zone 2).  I said sure and ended up taking the flat right away.  I moved in about 3 weeks later.

When I came over on a scouting mission 4 months before my move, estate agents were not keen on showing me anything as I wasn't ready to sign or move. 

Yes, you need to decide what area you want to look in and then see what estate agents serve that area. Brokerage type fees are paid by the landlord so you will not have to pay any sort of fee just to look at properties.  So, it doesn't hurt to see as many estate agents as you want.  Fees you might be charged when you decide you want a property would include a credit check, a check-in fee which is basically an inventory of what's in the property and what condition it's in and possibly some kind of application fee.  As I said before estate agents are not registered or regulated (only voluntarily so) so types of fees and the amount can vary from agent to agent but it shouldn't be more than a couple of hundred pounds.  However, you should be prepared to leave a hefty deposit.  Since you have no credit in this country, you may be required to pay 3 or 6 month's rent up front.

If your trip in August is your move over and not just a trip to look around, then yes you can contact agents a few days before are ready to look at properties.  As Kristi said, estate agents will have little interest in helping you until you are ready to move.

As far as giving you some piece of mind here are some of the voluntary associations that estate agents can be members of:

National Association of Estate Agents
The Ombudsman Scheme
The Royal Institution of Chartered Surveyors
The National Approved Letting Scheme