My advice for any thesis is to pick a topic that is interesting to you. Enjoying your research will make the work seem much more manageable. I have experienced both sides of this.

My undergrad training was in Classical Languages, and I was headed towards an MA thesis on either Ovid or Aristophanes. I wasn't really thrilled about either at the time, but that's where my momentum was taking me. Than at the last minute, I had a class on Carthage and decided to do a 180 and write an archaeology-based thesis on the Western Mediterranean in the 8th century. I loved it, and voraciously consumed sources on the topic. I learned about Carthage and the Phoenicians in the West, about Greeks in the West, etc. Loved every minute of it and had no trouble generating a thesis.

Fast-forward [a lot of] years and I'm through PhD quals. It's time to get really moving on a doctoral thesis and again I've got a certain momentum of where I should/will go. My advisor(s) have their special areas and I feel I should utilize their expertise. The topic forces itself on me in the end; the problem is that I've become less interested in that topic. This is not good. As I began the doctoral diss, my experience was exactly the opposite from MA diss. Every step seems onerous, every paragraph is a chore. Instead of devouring articles and books about my topic, I curse every time a footnote gives me another lead to track down.

So bottom line: find something you want to learn about. A thesis is basically an opportunity to dive into a subject area like you have (probably) never done it before.

Secondary advice: make sure your topic is viable before you start. Your advisors will help you with this part. If they don't, be sure you can answer the following:

1. What is your question? You need a question.

2. Why are you asking this question? What is the problem?

3. How and where will you look for an answer to this question? Is there primary source material?

4. Do you have an idea of how the answer could be structured? Will you organize by conceptual topic? Chronologically? Etc

5. Before you begin, what is your basic hypothesis. How do you think the thesis will play out [this usually turns out wrong in one way or another, and that's ok]

6. What are the stakes of answering the question? Who will care, and why should they care? In what way will the answer to the question change the way [topic] is understood or perceived?

I cant give any advice on masters thesis, but I can describe my process for picking my undergrade thesis topic. I know my undergrad research isnt quite the same as masters work, but describing my process for picking a topic may help a bit.

I was in a class called "China under Chaiman Mao" and wanted to learn about Chinese environmental conservation, so I looked into some early parks and reserves. I found a large amount of early protected areas were designed to protect Giant Pandas, so I started to investigate why. I then kept digging on the history of pandas in modern china. They're were a few secondary sources on the origins of Panda conservation, but not many. Eventually I wrote it on how the PRC used Panda's to influence foriegn policy, which I had not found much previous writings on (stuff like panda diplomacy and working with the WWF).

I started with a topic I was interested in (conservation) then I found something I thought was interesting about it and researched why it was that way (tons of early panda reserves) Then I found "holes" in that topic I thought I could fill with my own research.

I wrote mine on the relationship between Uncle Tom, Uncle Tom's Cabin, and the antebellum African-American community. I argued that Uncle Tom, novel and character, came up against intellectual traditions/continuities within the AA community that stretched back decades.

I stumbled onto this topic accidentally. I was interested in antebellum history, particularly in the South, and the sectional crisis broadly. As a first semester MA student I went to the Southern Historical Association annual meeting, this time in Atlanta. On a spur of the moment decision I went to go hear Juanita Abernathy, civil rights activist and wife of the late Ralph David Abernathy, talk. In her talk she mentioned how some people accused her husband and Dr. King of being "Uncle Tom's." This spurred me to think more about what an "Uncle Tom" was and my research project started there basically.

If I could go back and do it all over again I might do it differently. I would definitely have kicked myself into high gear with regards to working on it a lot earlier. I may have also looked at a different topic. I just finished up my first year in a phd program so I am beginning to explore dissertation topics and I am leaning towards the changing rhetoric of abolitionists during the sectional crisis, particularly African-American abolitionists. However, I am also somewhat interested in Europe during the World Wars and the interwar era. I may write some articles in that but who knows.

Like I mentioned earlier I wish I had gotten started on it earlier. It definitely helped me mature as a person and as a scholar/historian. Alkibiades had some very good tips as well. Basically your thesis, if you choose to go to a phd program, will occupy your work for about a decade or so. Make sure it is something you like and find interesting.

You should read this old thread, several comments on choosing a dissertation/thesis topic. :)