It cannot be said enough that planning is an essential step to the writing process. Whether or not you're given a topic about which to write, sit down and plan your paper out. Organize it by thesis statement/introductory paragraph, body one, body two and conclusion. If you need to, use brainstorming to cluster thoughts together.
Opinions are impossible to objectify. Simply stating that "cold weather is bad" leaves the reader wondering what the writer considers "bad," and how he is going to prove such a statement. It also takes away any chance of objectivity that the reader has. This is not the way to attract readers to a position.
Many times you won't have a choice regarding your thesis topic, but if you do, capitalize on it. Find something to write about that you can invest your time into without growing tired or bored. Also make sure that you have enough material or can find enough material on the topic.
It's hard to prove an argument that is vague or oversimplified. For example, making a thesis statement that candy bars cause heart attacks is almost impossible to prove with any certainty. Saying that a specific candy bar raises the risk of chest pain would significantly raise your chance of coming up with a viable argument.