Keep the idea fresh and original. You may get tempted to copy from the works found in libraries and off the web, and while it is healthy to refer, copying is a downright NO! A fresh topic leaves you with a lot of scope to express yourself and that is what will work in your favor.
Choose a topic that you can relate with. Remember, choosing the topic is not a one-time affair. It is a wedding and you will have to stick with it, so choose a topic that you know you will be able to write on, with ease.
Choose to take help from the right resources. Friends may help a few times, but you know you can never be sure about the accuracy and the authenticity of the help that they give you. Instead try getting in touch with someone who knows how to actually write a thesis and take their help in choosing your topic.
Keep your synopsis or proposal relevant to your topic. Staying in the introductory notes might indicate a prospectively loose thesis project, no mentor would approve of a topic if you do not concentrate on the quality of the proposal.
Remember the take of is the most important for a pleasant flight!