I'm not sure how helpful this will be, because I've never had much trouble finding places to put my votes. I think about half my interest in se.math is in answering questions and half is in reading other people's questions and answers. So I spend a lot of time browsing through questions and opening the ones that seem interesting.
Often I see questions that seem to be in my areas of expertise but which I don't know how to solve; I open those and see what's going on, and usually upvote at least the question. Often I see questions that I do know how to solve, and open them even when they already have answers. Sometimes there are better answers than I would have given, and I upvote those. Sometimes it seems to me that the answers omit some important issue or miss some interesting approach, and then I write my own answer. Often these answers are quite successful even though they are later than other, upvoted answers.
Fairly often I see a question that is so interesting that I instantly stop to think about how to solve it. If I stop for more than a few seconds, I upvote the question.
I like mathematics, and reading about mathematics often makes me happy. Any question or answer that makes me happy will get an upvote. I rarely downvote.
I have a few tags marked as favorite tags, and I read through most of the questions that come along with those tags. The main one is logic. None of them are very high-volume, so following most of the questions is fun and not onerous.
I upvote any question that makes me pause and say "Oh, I've always wondered about that," or "Oh, I'd like to know the answer to that." Then I upvote the answers I find helpful or interesting.
If the question shows that the poster made an unusual effort to make it clear, I upvote it. For example, today someone asked a very simple question about a circle in a square. I would not normally upvote this, but they took the trouble to include a diagram in their question. I upvoted the question because of the diagram.
Often I open a question planning to post a certain answer in mind. If the answer is there already, I upvote the answer. If not, I write it up and post it.
I upvote any question that attracts a lot of dispute in the comments about what the correct answer is (for example), or that attracts a lot of wrong answers.