Invoice them at your normal rate, and aim to get paid straight away as per any other one-off customer.
Watch out for promises of lots of work and excuses as to why they can't pay you straight away. Like all things there's good and there's not so good agents, so try and build a relationship with them, but that will take trust on both parts and time. Doing a good job, at a fair price is the best way forward, but there will always be those who don't want to pay your rate, or stall on payments. I do work for several agents and sometimes have to chase them a bit for payment, but they've mostly stopped asking for a price first and just tell me to get it done.
Like Neil says, there's always the one that uses a really cheap guy and will ring you for an emergency job, when he can't get hold of him, or as I've found, one particular agent that occasionally rings, and it's usually because they've got a really fussy landlord or a problem carpet, suprisingly, I find I'm usually fully booked now when I get that call
