1) alevels are great if you keep on top of the work from day one. Not excessively, but go over your notes, revise topics as you do them. Anything you are still forgetting or struggling with, repeat more frequently. That way when exams come along you know your strengths and weaknesses. Use different techniques too, not just note taking, this supposedly increases your memory pathways to the information so you can recall it easier
lots of people I studied with said they had to really tweet their revision technique from GCSE for alavels.
2) personally I got as much of the work experience done in first year, Surrey for example only accept work experience up to the date of application, so you only have until the beginning of second year. Get your lambing done early, some farms lamb over Christmas so start looking now.
once you've got one farm, provided you work hard and are enthusiastic they can normally refer you to a friend or family member if you are struggling to find another placement.
3) try and get yourself some unusual placements and ones that show you've really considered the whole career. Abattoirs, labs, charity work.
4) Ask lots of questions and go away and look things up. For example on dairy, I have a dairy vet book and I look things up in the evening and ask questions. If you don't know the answer on placement say so, one of the comments I've heard from farms is how they hate it when students pretend to know the answer. They'd rather you said you didn't know, then they can teach you. This also goes for if you are asked to do something you aren't confident on, or are unsure, always better to double check, or get help with it. None of the farms I have been on have begrudged showing me again, or confirming information again. They would prefer you got it right. Also, if you do get something wrong, be honest and don't freak out. Accidents happen, and even if someone is annoyed at the time about it, things move on very quickly. It will soon be forgotten.
Enjoy yourself too, the placements should be nice relief from the day to day studying. I found they helped keep me focused and motivated!