An easy way to develop your thesis is to make it into a question that your essay will answer. What is the primary question or hypothesis that you are going to go about proving in your paper? For example, your thesis question might be “how does cultural acceptance change the success of treatment for mental illness?” This can then determine what your thesis is - whatever your answer to the question is, is your thesis statement.
Any information that doesn’t fit within the framework of your outline, and doesn’t directly support your thesis statement, no matter how interesting, doesn’t belong in your research paper. Keep your focus narrow and avoid the kitchen sink approach. (You know, the one where you throw in every bit of interesting research you uncovered, including the fungal growth in the U-joint of your kitchen sink?) Everything you learn may be fascinating, but not all of it is going to be relevant to your paper.
Although it is tempting to simply read over your essay and use the spell-check tool, editing your paper should be a bit more in-depth. Have at least one, but preferably two or more, person/people look over your essay. Have them edit for basic grammatical and spelling errors as well as the persuasiveness of your essay and the flow and form of your paper.
If you are writing a research paper for a class, consider the other students. Is it likely that they will also be writing about your topic? How can you keep your paper unique and interesting if everyone is writing about the same thing?