As a famous saying by John F Kennedy goes, "Do not ask what your country can do for you, but what you can do for your country" holds good everywhere.
The scientific community is mostly very well knit amongst like-minded helping people. However, there are some exceptions. Most of the things, work out of recommendation, in addition to proof of one's work in the form of publications and manuscripts.
When I say, recommendation, it's the recommendation of the seniors such as senior scientists/mentors, laboratory heads, and sometimes your peers, as well. Mostly, people/scientists who hire other junior scientist ask for 2-3 recommendation letters from the senior scientists/mentors with whom the junior scientist/researcher has worked. Once a researcher finishes her/his tenure in one laboratory, moves on to work in different laboratories. Usually, the move of a junior researcher to another laboratory in the scientific community takes place due to a change in location and her/his movement to the area of preference, or else, to a laboratory of a higher significance/repute. In either of the cases, it is vital for the junior researcher to have a backing up with a good recommendation letter from the previous workplace/mentor/boss/senior.
The trick in getting a good recommendation letter is straightforward. A person is judged on his/her day to day dealings, in addition to, the scientific aptitude, attitude, work ethics, discipline and helping nature. For all the above mentioned, the person should have contributed enough to earn the points. Many of the times, interns come for a brief time, behave over-ambitiously and try moving to another laboratory of good repute. In a lot of instances, such short timers over-represent their contributions and training/learning and try shortcuts to get a good recommendation. However, such behaviors are highly discouraged, and often such recommendation letters can be counterproductive, as well. On the other hand, any long timer (at least two years), who has contributed not only to his/her growth, but the growth of his/her fellowmen in the laboratory, as well as, towards the research goal of the mentor is given a good recommendation letter.
Finally, the bottom line is being selfish does not fetch you anything, and instead can be counter-productive. The mentor/boss is smarter than the junior researcher, and is always watching! Last, but not the least, it is advisable to evolve oneself as a real scientist with a real passion to do science, and not for the sake of show-off. Finally, simplicity, helpfulness uplifts every individual and pays you in every way in the long run.
