Some profs provide sample answers on file. Some profs invite you to bring your answer to office hours to go over it with you.
For me, this is where study groups come in - with 2 or 3 people going over a problem, you're more likely to hit most of the issues.
Also, for 1L, I used Seigel's Essay Exams books - tons of essay exam questions w/ sample answers. These were probably the MOST helpful b/c it gave me a good example of how to structure my answer for each subject beyond the more general system of analysis I got when I used LEEWS.