Here's the best way to cook it, so it isn't slimey. Cook all of it at once, and freeze it zip lock bags, enough for each training session. Or, in my case, a ziplock bag for each weekend of shows.
Start with a large pot of boiling water. If it isn't boiling, when you put the liver in, it will stick to the bottom. Take the pieces out of their packages and rinse really well under water to remove any icky red liquid. Rinse well, because it will keep the pot from foaming so much. Give the water a generous shake of salt and several chopped up garlic cloves. Drop in the liver, and gently stir the liver to make sure it doesn't stick. Keep the water at a gentle roll for about 15 minutes. You may need to skim off foam from the top. I use a coffee cup and slide it under the foam to catch it.
Pre-heat your oven to 325. Line a cookie sheet with foil. Remove the liver from the water and pat dry with paper towels. Place on the cookie sheet and bake for 30 minutes, flipping over half way through. Remove, cool and place in baggies. Into the freezer. This technique keeps your pockets and suits clean.