Finances have a huge role to play in the progress of a PhD. An absence of financial pressures can help relax one’s mind and focus more easily on more intellectual pursuits.
Even if one has full funding for the PhD (which most PhD students have), the stipend can at times be minimal. Many PhD students work part time to get some savings or make ends meet. During the last three years, I have also worked part time.
Main priority is the PhD
For PhD student thinking of working part time, my first suggestion will be that if you are not under financial pressure to work part time, do not. It may be much better to squeeze time to solve some research problem which may be much more useful in the long run than savings for your next Euro-trip. That new result might be the tipping point to land you your ‘proper job’.
Find relevant work
If one does need to work, it is better to look for work directly aligned with your area of study. For example an Economics related summer internship for an Economics PhD student may even help in his/her academic profile. Moreover, it is useful to get teaching experience in one’s own department.
Sometimes, one may want to switch off one’s mind and want to do work on something different. For example, I have greatly enjoyed tennis coaching as a part time job. Since PhD is a thinking game, it is better to seek a low intensity part time job where one can still be able to think about one’s research.