The below tips were (sadly) learned from experience. Pretty sure I did nearly all of these things wrong on my first attempt at baking a birthday cake.
1. Cream the butter alone. Then add the sugar and cream together. This is a genius tip from my mom that makes a big difference. And, also: cream the butter and sugar together until there is no gritty feeling remaining. That's harder than it sounds; even with a hand-held mixer, a person can get bored of mixing.
2. Add the eggs to the creamed butter & sugar one at a time. For maximum aeration!
3. Don't overbeat. Once you're at the stage of mixing the dry ingredients & milk into the butter/sugar/egg mix, mix ingredients just enough to incorporate.
4. When you're alternating dry ingredients and milk, end with the milk as your last step. I assume this means start with the dry ingredients.
5. Prepare your pans correctly. The parchment paper? It's worth it. The butter? Use as much as you can. And then more.