One tip a professor gave me is to ask, "Would you be able to give me a positive letter of recommendation?"

You don't want just any letter, it needs to advance your cause.
It seems like you would not ask somebody you were not sure of, but people do. As you said, if the person is less than enthusiastic about writing the letter, you don't want that letter!

Great point! Thanks for adding that

This is not only true for grad school, but when you look for work too. Before you use somwone as a reference ask them if it's ok. If they don't look like they are happy, you don't have to withdraw your request, just don't list them.

School is an excellent time to learn the value of a good reference!

Here's the article for those who are interested -

Letters of Recommendation Tips

My thought is going to be to get the letters while I'm IN the class. If I find a teacher I adore I'm going to ask them for a letter of recommendation. That way I have it on file. So far I haven't hit a perfect combination of class-in-my-field and teacher-I-love, we'll see!