Recon: Farms are dangerous places, there are plenty of sharp implements, inspection pits etc. It pays to check out the area you are going to shoot in daylight. This also allows you to see rat activity signs, where they are most likely to be feeding and where they are coming from... this allows you to set yourself out some ambush points!
Bait: Set out a distraction in your shooting zone, rats love sweet stuff and a mix of peanut butter, chocolate spread and vegetable oil can prove deadly as they tend to sit and lick it off whiskers and feet giving you enough time to draw a bead for a clean head shot.
Accuracy: Rats are tough little suckers and I've seen them run off after being hit incorrectly in the belly with a .22 rimfire! The chances will be that you're using .177 or .22 air rifle calibres so head shots for a clean kill are a must. Practice shooting into a small head size target out to 30 yards or so, and know your aim points from 15-35 yards as rats have a tendency to pop up where you don't expect them.
Night vision: Rats get fly to torches and lamps the same way rabbits and foxes do. They can be a bit more relaxed under security lights and lights that are in use all the time on the farm but flick something new at them and they can get skittish. Night vision is becoming much more affordable these days and I have had some great ratting forays using the NiteSite units.... silent to use and will really give you the edge when it comes to the rat job.
Tidy up: As with any type of shooting, rat shooting needs to be cleared up before leaving the site. ALWAYS use gloves to pick up dead rats as they carry a multitude of potentially life threatening diseases. Also, shooting in the dark means that you might miss a few so give it an extra 5 minutes and make sure you've got them all.
Discretion: Be discreet, not everyone wants to admit that they have rats around their buildings even though rats and farms go hand in hand due to livestock and also a readily available animal feed source. Don't be telling all and sundry exact locations, names of the farms you shoot.... it'll make sense in the long run!