you still want a nice looking girl with a good attitude which sadly most of these wannabes don't provide
But you have no way of knowing that til you book them, other than to read the testimonials...
As to 'nice-looking', look at their portfolios - you should be able to tell from the most recently-posted images what's what, surely...lol
Like Ali says, read the recommendations from other photographers carefully - weed out any no-shows - check that they're updating and checking messages regularly - like every 48 hours or so...
After booking make sure you maintain good comms - either by email or SMS right up to the morning of the shoot.
Make sure you know what you're doing and that she knows what's expected of her (again I'll assume a female model for sake of simplicity).
Make sure you take money with you - having to go to a cashpoint because you forgot to bring enough cash is unprofessional.
Provide rest-breaks and food/refreshments if working through meal-times. Models need food, despite what the tabloids have us believe...
Don't assume a 4-hour shoot means 4 hours of continuous photography. Allow time for make-up and hair, as well as 'comfort' breaks, food and refreshments.
For the model, the clock starts the moment you shake hands, not the moment you press the shutter for the first time. From a four-hour shoot you'll get about two and a half hours of usable photography if you're lucky and work fast.
A model is in effect your employee for the duration of the shoot and you have a duty of care for that employee...some blokes will disagree vehemently with this, but they're bloody wrong and mostly GWCs anyway...
You'll get 'burger-all' in the eay of good images if the model thinks you're an idiot/perv/incompetent...so be professional.
You might want to lick her all over like an ice cream and she might well have that cheeky glint in the eye that's saying 'go for it' but it's an act - that's why it's called 'modelling', plus it's generally frowned-upon...