The first thing is to make sure your face is washed and clean.
Toner. Many people don't use a toner and I don't think it's completely necessary everyday, especially if you use a pH balanced, gentle foaming cleanser. However, for masking, it it effective to use a toner so the serums from your mask absorbs into your skin easily, and effectively.
Time. It may seem like a waste to remove your mask full of essence, but honestly, it will reverse effect if you leave your mask on longer than what is directed on the directions. Usually, the range is between 15-20 minutes. I usually keep my mask on for 20 minutes. Leaving your mask on any longer is not good for your skin! Your mask will start to suck the moisture OUT of your precious skin.
Neck. Your neck is precious too! It's like eye cream. You know you need to use it because at the first signs of aging, your eyes and neck are the first indicators! A little DIY never hurt anyone!
Essence. You will most likely have extra essence left in your bag. Sometimes, if your sheet mask included a plastic safety sheet on your mask, when you squeeze it, more essence will come out and you can use it on your face as well. Or use the extra essence anywhere else you feel needs some extra moisturizing, like knees and elbows.
If your mask is more pricey ((i.e SKII or Shiseido) and is especially full of essence, I sometimes squeeze the mask while it's still in the bag while taking it out, and you can store your extra essence into a dark dropper bottle and put it in the fridge. You can even put your mask back into the packaging and seal it closed, put it in the fridge and save it for another day. Your one mask could become 2!