The sticky about anesthesia about covers it all:
http://www.yorkietalk.com/forums/yor...-protocol.html
Some additional considerations: make sure your vet is very experienced in small dog surgery. Have the blood work done a week ahead of spay, the more comprehensive test & a little more expensive (CBC & chem panel) they send out, rather than an in-house blood test on day of spay which is limited.
Be prepared for after care, keeping them quiet, restricting jumping etc., know how to protect site from licking, and if the vet says pain meds are optional take some home anyway just in case you need them in the middle of the night.
Having them in surgery will be one of the most stressful days of your life, but hopefully it will all go well and they will be home recovering in no time.