This can be a fightening subject for many PA school applicants. The key to getting good letters of recommendation is to start building the necessary relationships in advance to the application deadline.
Most schools requre two or three letters, one of which must be from a physician or PA.
In these letters, the schools want to know about your personality, your dedication, and your ability to work as part of a team. Are you reliable? Are you cooperative? Are you organized? Mature? Committed to the profession? Do you have people skills? Are you a good student? A good coworker?
When selecting letter writers, it is important that you ask people who know you fairly well... and seem to like you!
Asking a professor that you have only interacted with in a lecture setting is risky, because he may not know enough about you to write a sincere or helpful letter. It's actually bad for your application if your letters of recommendation indicate that the writer knows little about you. It's especially bad when a writer expresses how you'll make a great MD. Obviously they don't know you well enough to know your life plans....
For those who do not work in an environment with a lot of doctors or PAs nearby, it would probably be a good idea to do some (or a lot of) shadowing for a while before you need the letters. This will help you in other aspects of your application, too.
For those who don't know any professors that well, consider TAing for a class you liked or researching with them... even if you don't need the money or credits.