I take my first SG reading right after I stir everything up and right before I pitch my yeast. I take the next SG reading about a week or 10 days later, to ensure fermentation is finished. I either rack it then (like if I'm dryhopping in the carboy), or wait until I'm ready to rack it. Right before it's bottled or kegged, I double check to make sure it's really finished.
I have always used a sanitized, new turkey baster for pulling my samples.