Trying to find a new job is complicated and stressful. Writing resumes, identifying a position that fits the skill set one is best at is tough enough without all the uncontrollable issues. Once one submits the job application, there is little more to do but wait and see if an interview is in the offing. But there is one more thing one can influence; reference checks.
Of course there is no way to control the conversation between the perspective employer and either the old employer or the coworkers and friends the job seeker has selected. Still, it does not mean that one can not influence the interaction. The approach is different between the boss and coworkers.
Under ideal conditions, the departure from one job and on to another is done in a calm and mutually respectful manner. Unless there is a vitriolic outburst leading to a precipitous and bridge burning termination, it is worth the time to talk to the current boss about his reference intentions. Even if he is not really happy to give you a great reference, it is possible to request he not deride your performance.
If the boss does not want to help you put because he does not want to lose you, you can appeal to his sense of fairness. Your decision to go a separate way should not tarnish the solid performance you have given to your work so far. If he does not think you were a sterling worker, perhaps you can convince him to just give a noncommittal generic reference that you did your job as expected.
Selecting coworkers can be tricky, as many employees do not want anything to do with employers if they can help it. If they respect you, you can help them out by letting them in on what the critical features of the new opportunity. Giving them a picture of why the new job is a match for your skills will make it easier for them to help you out, and more likely that they will.
As far as how you help the process for your potentially new employer, it comes down to convenience. It is important to make sure the contact phone numbers and email addresses are current and accurate. It is also possible to actually provide prearranged written references mailed from the reference to the new employer.
There is also a concern about overdoing it to be cautious about. Having references write out statements that are over the top in describing candidate capabilities can backfire. Employees know that selecting your friends and having them prop up your skills is not difficult to do, employers need looking for reality and honesty.
The reason new employers look to reference checks is to at least be able to assure themselves that a new employee was not more problematic than helpful at the last job. Coworkers are anticipated to say good things about the candidate, and being unable to provide them is a negative indicator. If a job candidate does a little work to help the process out, it can have a positive impact on the job search.