* I like tight clusters, ones that don't move, so I tend to wiggle the top rondelle down a little and hold it in place with my pliers as I make a closed loop. As I wrap the loop closed, those wraps hold the cluster in place.
* If I'm making cluster earrings, I do each step for both sides before moving on to the next to make sure they are even.
* I like 4mm rondelles the best. 3mm is a little small, so you might need more trios to make this look nice and full (and not droopy). 5mm rondelles may need only 4 trios instead of 5 - it depends, so have extra supplies on hand as you work.
* I like mixing in other shapes, too, like tiny brios, wheels, rounds, and buttons - it gives the cluster a nifty and different look.
* I like 26ga headpins for rondelles because some just wouldn't take 24ga. Rather than have a ton of each size headpin on hand (since I made my own), or manually drilling out each hole (not fun), I went with the smaller size. Because I wrap them tight, they remain secure, and no stress occurs on the loops from constant movement. If the rondelles you're working with will fit 24ga, and you'd prefer that size, go for it.
* Similarly, if you prefer using a thicker gauge for the headpin, like 22ga to 18ga, those work just fine, too. The only change you might have to do is make the third loop a little wider to accommodate the bigger gauge (this depends on how small you make your loops in the first place).