Choose a research area that you are passionate about. Regardless your field, you will have good days and bad days. On good days you will be enthusiastic and motivated to work.  On bad days, you might question whether your research makes any sense, and you might even doubt your ability to graduate.  If you pick a meaningful topic, the daily setbacks in your research will not bring you down. You will still be working in an important field, and you will be learning the skills and expertise necessary for your career.

Pick a topic that can be completed with the available resources in a reasonable amount of time.

Some students make the mistake of being too ambitious in their thesis plans. A good rule of thumb is to expect things to take three times longer than they would if everything went smoothly. Talk to your advisor about choosing a topic that is novel, realistic for your time frame, and can be completed given your resources (e.g. equipment, expertise and money).

Look for projects that incorporate marketable skills

Graduate school can seem (and feel) like eternity, but there is a light at the end of the tunnel.  What will you do after you graduate? It is a good idea to look at job postings in your field to find out what employers are looking for, and then design a thesis that will prepare you for those types of jobs.

Have several backup plans in your thesis proposal in case you reach a dead end.

A former graduate student who finished his thesis in 4 years (when the average for his department was 5-6 years) attributed his success to a long list of backup plans. "I had at least three backup plans for every experiment. There were times when I had to resort to my third backup plan before things I got anything useful."

Planning is crucial to success in graduate school, and the sooner you start visualizing the progression of your thesis and publications, the sooner you can get your thesis and career on track.