Layer 1: Blind Post
This is the most legitimate GET of them all. You come to 4chan with absolutely no idea what's going on, post somewhere, and, BAM, you got a GET. Your post is just some random post in a random thread, with or without a picture. These GETs are mostly deemed as failGETs, but without a shadow of a doubt, these are the most legitimate.
Layer 2: Post Number Monitoring Through Page Refreshing
This is when you're waiting for the right time to post for a GET, so you refresh the page over and over again. Then, when you believe the post count is correct, you hit Post and get the GET. This is slightly less legitimate than Layer 1 because you refreshed the page to monitor the post rate, giving yourself a greater advantage than someone posting randomly. Does this make this GET illegitimate? Not at all. In fact, this method is the method used by the majority of people going for GETs.
Layer 3: Post Number Monitoring Through A Post Number Updater
You can use either an in-browser greasemonkey extension (the method I personally use) or an external program to monitor the board's post rate. When the number reaches the general range you want, you hit Post. This is less legitimate than Layer 2 because you are going for the GET with the aid of something beyond refreshing. Whether you wrote this program or not, THE PROGRAM instead of 4CHAN BY ITSELF is telling you the post count. Does this make the GET illegitimate? People have been arguing this for months now, but the general consensus is that no, it's not. YOU are the one hitting the Post button, so it's still up to you to correctly predict the timing of when to post. Update lag, server lag, jumpy Internet connection, and overall random influx of posters can severely detract from this method, but if you're really good, this method is the best method to use.