I normally only come up with them right at the end of the year on silvester although I often have a vague idea already. This year, I'm really not sure yet.
For one, I think I'd like to keep my wordcount goal which was one million this year.
Then, I'm thinking of doing two projects that would be related by theme and might make interacting with readers more fun.
I also think I'm going for at least 6 publications the year again but I'm not really sure there yet since I still have several long projects ongoing and the last novel took four months because I could only do weekly releases. So I'll have to think about this a bit more.
As for how I make sure I reach them: I write them down on a piece of paper and pin it to my whiteboard, then look at it every now and then and check off what I managed to achieve.
For the wordcount goal, I have a calc file where I put in my daily wordcounts so that I always know how much I've written so far. The monthly goal was 100k words per month this year which worked out fine other than in the month where I had to hand in my master's thesis. So I think I'll continue with doing it this way.