Note the Company Culture
Knowing the culture of an organization is essential to formulating your approach. If the culture is supportive of new ideas and changes, it will be a lot easier to start off. Patience is another crucial attribute for an organization to have in order for it to accept agile. Adoption of agile practices takes time and innovation. The organization should also not be terribly adverse to failure, because some tactics are bound to fail.
For most agile adoptions, you will need have support from the management. The more flexible the management is, the easier it will be to test out new ideas. Since the organizational culture tends to reflect its management, a flexibly managed workforce will probably be more receptive to agile than one with an overly strict management system.
At first you'll want to cater to the innovators and early adopters of the group. They will be the most receptive to change and their momentum should carry adoption over to other group members. Agile has a way of highlighting the problem areas (and people) within an organization. The change can be slow and painful if there are a lot of problem-employees.
Take it Slow
Start out as an observer and slowly start to suggest changes. Short, frequent meetings are a good way to start. Don't introduce a lot of new concepts and terminology early on - that can lead to fear and anxiety. The best tactic is to start by finding small problems and implementing simple agile fixes. Try and determine the pain points and anti-patterns in an organization by listening to the team members, management, and other stakeholders. It's likely that agile can address those areas and possibly win over the organization if successful. Hopefully, the organization will begin to see the benefits of agile practices and they'll start allowing broader changes. This understanding comes when you educate the team without pushing them. Presentations are one way to educate, but one-on-one hallway talks are equally important.
Try Starting a "Test Bed"
If the organization is large enough, try and implement agile within one of the departments as an experiment. Pick a team that has more innovators and early adopters so the transition has the best chance of success. If you have a successful adoption to showcase, it sends a powerful message to the management for a broader adoption plan.
Commit and be in it for the Long Hall
The organization isn't the only thing that has to try and change- you have to change as well. As someone who is trying to implement agile, you have to be committed to the long and arduous task of process improvement and all the ups and downs that it brings. The time it takes to sufficiently implement agile depends on a number of factors, and some of those variables are out of your control. Sometimes you have to be willing to relinquish control of the adoption to a higher management position for the good of the team.
Empathize and Adapt
Empathy is extremely important in any form of communication. You have to try and see the change from the developer's point of view. Listen to the team's suggestions and don't dismiss them just because they're unconventional. The things that you have read about and the things that you propose will not always work. Ultimately the organization into which you are introducing agile will be your guide. If a crucial practice doesn't stick, try and find a way to compensate for it. You can only introduce concepts that will work for them.