Tish,
Great advice from others - pigs don't like the metal of the usual P6 ramp,so carpet or rubber both work well, we used carpet but upgraded to a smallbit of rubber horse mat which also works well.
Sheep are domesticated from a mountain animal - so say boo to a sheep and it will run up the mountain and into a cave ,hence sheep are dead easy to load, just get them behind the trailer and go "shoo" they go up the ramp and into the dark trailer.
Pigs are a woodland creature, and if startled they run into a clearing so they can see their enemy (dark
wood to light clearing), and naturally run faster downhill so go downhill. Hence putting a pig up a ramp and into a dark trailer is so difficult - it is against all their natural instincts.
If you can park your trailer downhill so the ramp is as shallow as possible. If you are loading in the open, don't do this at the brightest time of day,as the trailer will be darkest, as Eve says load in the evening, and there is less difference in the light.
No problem with loading the evening before - in effect you are just putting your pigs to bed a bit earlier, If you do your abattoir trip early, then they will have slept in their "trailer" ark and then woken up to get out at the abattoir ! You can load as late as you like.