I don`t think anyone has 100% success when splitting rocks and everyone has there own ways. I`m a stonemason as well as a fossil collector so I hope I can help.
Some rocks split much easier than others. Some just will not split well what ever you do but there are tricks to try.
The first thing is to make sure you identify which way the bedding planes are running. This can be extremely difficult in some stones if there is no obvious lamination. Remember that you will want to split the stone along the bedding(ie paralell to the sea/lake floor) as this is the way most fossils lay.If there are any fossils showing on the side of the rock,however small, they should give a good indication which way that is. Some stones can be taller than they are wide or long which could lead you to split the wrong way.If you do that you will be breaking accross the fossils.
Grouped fossils showing in a band is a great giveaway and cross sections of Ammo`s means AIM HERE.
Personally I like to split using a chisel, as it is more accurate than the using the chisel end of a geo hammer, and strike it with a hammer with enough weight to do the job.The chisel needs to be of thick enough gauge so that it doesnt spring in the hand when struck so all the blow is transferred to the stone. Chisels should always be kept sharp but the angle of the blade is important.You dont want to cut or carve into the stone, you want a controlled break along the bedding plane. I have seen fine angle tools such as wood chisels used on soft shales like kimmeridge but try that on a hard lias nodule and you`re wasting your time. A cold chisel with an angle of about 60% at the tip would be better and it would still split shales.
Running the chisel blows around the stone until it goes often helps but sometimes only a part will split away and a stubborn stone can result in what i call a pillow where you are left with no flat side left on which to strike. Some nodules startout ball shaped to start with which doesn`t help. If you have revealed the edge of something good then take it home and prep properly with points etc as further splitting can often be a split to far. With nodules from the Eype clay beds, they dont tend to split clean around the Ammo and an edge is often the best you can hope for.
If there are no fossils showing then to carry on you must try to crack the stone arcoss the middle to give you a fresh side to split.Try to hit on the top but off to one side, not in the middle. A blow to the middle often results in it smashing.A blow to one side should result in a break from that side across to the other. If a cross section of a fossil shows inside you still have both sides and a clean break and it is possible to glue it back together and prep, or prep one side, glue and prep. If nothing great is showing then you now have two fresh edges to split.
When the pieces start to get a little small then I find a stunted "sawn off" version of the chisel can let you carry on when it starts getting fiddly then the chisel end of a small geo hammer to finish if after the small stuff.
I know others on this forum will have ther own ways based on what works for there rocks and I think it would be very interesting to here them.