If you are a few years (or more) out from applying, make sure you foster your relationships early on and build ties with the vets, professors, employers, etc as these are the people that will be helping to decide whether your application goes into the admitted pile or the rejected pile.
Say you are interested in a research project with a professor (not only will that look good on an application, but it will also allow you build a stronger relationship with that professor such that he or she will be able to write you an even better recommendation. Of course, that is assuming you do a good job, go above and beyond their expectations, and genuinely amaze them with your abilities and dedication. Remember, professors write letters of recommendation every year for many students; because of that, professors have in their mind a status quo, so to speak. People at that level are good, they do what is expected and are genuinely good students, co-workers, employees, etc. You’ll get a good letter of recommendation, but it won’t be glowing with positive remarks.
What will make you stand out in the eyes of a professor, advisor, employer, vet, etc is going above and beyond their expectations. So, when you enter a new situation, take note of what the status quo is — how hard do people work, how much effort do they put in, how attentive to detail are they, etc. Then take it up a few notches!
Also, it is perfectly okay to begin a new experience or relationship by telling the individual up front that you may want them to write you a letter of recommendation in the future or be used as a reference. This gives them a heads up to be tuned in to your behavior and work habits. As long as you can follow through, this is a good thing! They will note the extra effort right off the bat, knowing you very well may be asking them for that letter in the future. Mentioning this at the beginning of any new experience is also good because it lets the person know one of the reasons why you are there. Honesty is great and it keeps you both on the same page.
Whatever happens, make sure you go above and beyond the expectations of the potential recommender. They will notice!