Killing the specimen quickly and without damage is important. Most insects can be killed with a killing jar or insect collecting jar. A killing jar can be an empty plastic peanut butter jar with a cotton swab soaked in nail polish remover. Ethel Acetate is a chemical I prefer over nail polish remover since it kills much more quickly. Ethyl Acetate. is harder to find, but can be purchased at Insects4Sale.com.
Another way to killing the specimen is to place the specimen into a freezer for 24 hours or longer. This is preferred for small insects or insects with delicate wings (ie, Lacewing or Mosquito) that can get matted up in the in the killing fluid.
In the case of butterflies and moths however, gently squeezing the thorax of the butterfly between your fingers while it is still in your butterfly net works well. A problem with squeezing the thorax is that it can damage the specimen and make it difficult to spread. I prefer to inject the thorax with acetone using a relaxing syringe. This kills the specimen immediately and minimizes damage. The butterfly/moth can be transferred to an envelope where it can be kept until transferring it to storage.