Just got done investigating a spambot we had earlier, and it looks like they used a lot of compromised accounts on other instances to give their post an initial upvote boost. If you don't already, please remember to use a good strong password. Keeping your account secure helps reduce spam across the whole of lemmy, and keeps your account from getting banned for things you didn't actually do.
I recommend Diceware! I use it in my professional capacity as an IT/Security person, and also you get to use your mathrocks!
EDIT: Oh, also, all that numbers and symbols shit is no longer considered good practice. Just make it a really long collection of random words, at least 12, ideally 16+ characters. And make sure the words are actually random; your 3 favorite sports teams isn't good enough, which is why I recommend diceware.
I like the CorrectHorseBatteryStaple methodology.
https://xkcd.com/936
Basically what diceware does. It's just that humans are really bad at picking random words ("banana" is over represented, for instance) that's what diceware helps with.
I look around the room or think about what I'm doing. My username was made that way.
Not recommended. People can and do crib the kinds of things you're likely to have around you. It can narrow the field of guesses more than you'd think.