Before getting into technique, it helps to understand what you're actually doing when you steam milk.
Milk contains proteins, fats, and natural sugars (lactose). When you introduce steam, two things happen simultaneously: you're adding air to create foam, and you're heating the liquid. The proteins (mainly casein and whey) unwind as they warm up and wrap around the air bubbles you've introduced, stabilizing them into the fine, glossy texture known as microfoam.
The critical window is 140–150°F. Within this range, the sugars in the milk caramelize slightly, which is why well-steamed milk tastes noticeably sweeter than cold milk straight from the carton. Above 155°F, whey proteins begin to decompose: the foam becomes unstable, large bubbles form, and that natural sweetness disappears. Push past 160°F and the milk starts to taste flat or slightly burnt.
This is why a thermometer matters, especially at first. Stop steaming 3-5 degrees before your target. The temperature continues to rise in the pitcher even after you pull away from the wand.