Identify subjects that you find important. While certain topics may be timely or favored by your professor and other students, it's best to stick to topics that maintain your interest. Review class notes and assignments, pinpointing the topics that garnered the best grades and most positive critiques from the professor. If you've dealt successfully with a topic in class, you'll be better equipped to expand on it in a thesis than a more random subject.
Choose a distinct topic. You want the thesis to stand out even before the professor gets to the bibliography page. To do this, take a popular topic and give it a narrow and unusual spin. Students working on an astronomy thesis, for example, may choose to focus on the environment on the planet Mars and whether or not it can sustain plant life. This subject is too general and overused. Think of a spin on the subject, one that you can back up with facts and logical arguments.
Examine the available research on your chosen subject. Every argument or supposition the author makes in a thesis should be backed up by scientific facts, expert opinions from professional journals, graphs, charts and other documented evidence. Before officially choosing your thesis topic, do some detective work in the college library to determine if there is sufficient research material on the subject.
Meet with academic advisers. If you are having trouble deciding on a topic, or are unsure about subjects that will be suitable for your thesis, contact the professor or another school academic adviser. They can give you an unbiased opinion of your strengths and weaknesses in the course that requires the thesis. Take your adviser's opinions into account, but make your final decision based on your own interests.
Investigate prior theses that students have submitted. A student in a previous semester may have written on the same topic you've chosen. If so, read the thesis, if available, and think of ways you can explore the idea differently or add to the ideas expressed in it.
Keep a journal. Jot down thesis ideas that come to you in or out of class. This is instrumental in allowing creative juices to flow. It will also lessen the work you have to do when you finally sit down to write, as you'll have a rough outline of notes and ideas ready when you sit down at the computer.