I don't know if I can point you to guidelines for how to suggest edits. It's a fairly subjective thing, which is why the community approval process is in place to begin with. There are, however, a couple of blog posts with general information about editing that might prove useful:
Sorry I can't be more general, but I can be more specific. Looking through your recently suggested edits, I'd say you're not doing too bad the way you're going.
I personally like what you're doing to add a bit of context to answers. I have a habit of downvoting answers that contain only a link or only code without any explanation of how this helps to solve the problem discussed in the question. I consider those answers "not useful", and therefore deserving of a downvote. If I'm feeling particularly generous (or if the answer has significant potential), I'll make the same types of edits that you are. So I'd definitely be approving those. Not everyone agrees with me, unfortunately, but you can take comfort in the fact that one of the moderators who reviewed a couple of your recent edits does: Exhibit 1 and Exhibit 2.
What's probably motivating some people to reject your edits as too trivial is that you're not going quite far enough on some things people consider obvious. For example, we consider signatures, salutations, and "thanks!" to be noise. We encourage editors to remove these while cleaning up posts. One of your recent edits didn't do this, and I'm almost positive that's why it was rejected. If it had removed "Thanks in advance!", I'd have approved it in a heartbeat. (As it stands, I would have clicked "Improve", removed it myself, and implicitly approved your edit, somewhat like Gilles did here.)
This edit is a good one, and it got approved; same with this one. And this one.
This one's kind of a toughie. I agree with your intent and disagree with the rejection reasons. This should not have been a comment to the answer, and you were not substantively changing the meaning. However, I can see why someone would have said that it was "incorrect", or "too minor". In fact, it was a split decision (1 person did vote to approve). I probably would have clicked "improve", because a grammatical error crept in there. You didn't need the word "does"—"is" already worked just fine.
Your title edits are great—keep 'em coming! It's very difficult to write good titles, but they're very important on a Q&A site like this one.
Overall, I don't think you're doing anything wrong. It's just an inherently subjective process, and it's difficult to provide concrete guidelines. The reason that "too minor" is a rejection reason is because we want to discourage people from suggesting overly trivial edits because we get a bunch of suggested edits on Stack Overflow (so many that the queue often fills up and blocks additional submissions temporarily until we get a chance to get caught up), and because approving each edit requires the eyes and attention of at least 2 high-reputation users who could probably be contributing to the site in other ways.
You just have to use your own judgment about what is too trivial, but as long as you're making the site better, I think you're on the right track. Not too many more approved edits (or upvotes on great answers!) and you'll have full editing privileges yourself.