(1) If there were fewer agencies, they wouldn’t be cutting each other’s throats on rates, and expecting the drivers to limbo that low just to pick up odd shifts here and there.
(2) Pretending that one agency is any different from another needs deeds straight out of the gate and not words. Treating the driver as a “valuable asset” rather than “an expendable one” is a good start.
(3) To facillitate (1), don’t start an agency in an area thats flooded with them already, unless you’re offering MUCH higher rates. Not likely, so let’s move onto facillitating (2) which requires a new agency to do the things well that others don’t do at all. Such as “Automatic PAYE without quibble” and “Minimum rates of £■■” rather than saying £8-£15ph when you know damned well the £15 is only achievable on bank holiday overtime on the 29th february, and similar blue moon occasions.
The DRIVER is the customer first rather than the client. Drivers getting work off you will be back for more. That’s the bread and butter of agency business. Clients getting work done by you will be looking for someone else to do it cheaper, if they don’t care about quality, or will be looking to recruit as full timers those premium drivers that might come their way. What affordable T&Cs does your agency have in position with regards to retention, work-off contracts, etc.?
(4) Pay holiday pay, rather than deducting it from wages. Pay stamp, rather than expecting drivers to pay yours as well as theirs! 
(5) Don’t force drivers onto Umbrella fiddles, just because you make more commissions from that firm of accountants like De Poel rather than the honest graft of the in house crew you’ve built up.
(6) Pay promptly.
(7) Offset all fair expenses, without charging accountant fees.
(8) Don’t continually pester drivers for doing certain work when they’ve already expressed a preference for certain shifts, times of start, yards they like, etc.
If you constantly give the job on driver A’s doorstep to driver B who lives 20 miles away, then driver A is likely to up sticks, and move on in short order.
Drivers do talk among themselves, and there’s often some geriatric shooting their mouth off about “getting all the cushy jobs” because he’s allergic to certain types of graft, such as “any”. 
The “Availability” paradigm includes, or should include, a driver’s proximity to the job, and their keeness to do that particular job.
(9) Have an easy-to-follow pay structure that has a fairly narrow range, thus drivers can be assured of some minimum rate regardless of the time of day, type of job, or type of vehicle being driven.
(10) Above all, Don’t lie to your drivers - especially nowdays, when more drivers are wise to being bulled around by the vast majority of agencies out there, especially the ones who don’t specialise in drivers. To an “all rounder” firm, a driver is likely to just become more minimum wage fodder that needs to be pushed back in that direction at the first opportunity. 
