I think the key to being successful at teaching any topic, but particulary one that students are reluctant (I'm being diplomatic!) to learn is knowing your motivation. Why is it important to you and can you truthfully and specifically answer "Why do we have to learn this?" without resorting to "because it's the curriculum" or other such responses. If you don't know why you are teaching it, then they won't know or care why they are learning it. I think the poster who talked about bringing in a former student was on the right track. Maybe it worked in past eras, but the nature of today's students is that they cannot be taught in a vacuum. Learning for it's own sake is beautiful and precious, but is a skill that students must come to on their own. First we have to show them why and how English not only important and neccessary, but beneficial to them. The more real world type assignments you use, the more this will make sense to them.

I have discovered that one of the best motivators in the classroom, for anything really, is getting students to like the teacher.

Once I had a good reputation at my school--not for easiness or even niceness--once students started to really like me, they seemed more motivated to do almost anything.

Building a personal repoire with your students is one of the best things you can spend time on as a teacher.  Think about your favorite teachers of all time.  Wouldn't you have pretty much done your best on anything for them?

One of the things I did to motivate my students was to show them examples of adults who had not learned proper English.  I would pass around a business e-mail (from a friend's business, not from the school) with the names and proprietary information removed.  They would see how unprofessional and down-right silly the person looked when they sent a business e-mail that was filled with  grammatical errors and mistakes.  I would also show examples of what we were working on.  For example, I showed my seniors examples of bad resumes.  They were much more motivated to learn the proper English rules when they saw the outcome and consequences of not learning them.

Finding something that students enjoy reading is often a catalyst for their eagerness to learn English.  Another motivator are American movies and songs.  In Europe, teens ask Americans what something an actor said in a movie means, or something in a song because American films and songs are prevalent.

As mentioned in post 1, a guest speaker is an excellent idea to motivate students to learn English. I have former students come in and talk about the work force and how learning the English language helped them in their careers. One student in particular is a policeman. He tells the students how much he hated to write. Now, he is thankful that I offered him many opportunities to write. Now, he uses the skills he learned everyday as part of his job. This is one way to motivate students to learn English. 

I think the best way to motivate student in any subject is to make it relevant to them.  As a chemistry teacher, it is often hard for students to understand why a certain topic is important to them.  I try to use lots of examples in class that show why you need to understand something.  If they can see how they will use the language in their every day lives, then that might help. Having them do every day tasks (ordering food, dealing with customer service rep, etc) might help them see the relevance.

 My immediate thought when I read this question was that it pertained to students who do not read or write English well because it is their second language. Such students were often placed in a regular class because their speaking skills were reasonable and they interpreted for their parents. However, speaking requires different skills than reading or writing.  Just as I could not catch the exact sound or look of a word in Spanish which I showed them, they were not catching the endings of words in spoken English. Knowing that I had the same problem helped them see it could be overcome.  What appeared to be reluctance was actually a lack of understanding. Reading aloud to them helped.  Working in carefully created small groups helped.  Using Kagan techniques in that small group forced each person to be responsible for an answer or a sentence to read to the group.  Each small step built success which led to a better attitude and eventually to the motivation to do better. Written notes mailed home for the smallest positive helped.  I agree that mixing it up helps such as students writing questions on cards, walking around the room to exchange them, and giving answers to each other.  Putting teacher created cards on students' backs and the student trying to guess what the card said by other students asking them questions about the topic without telling them the actual word.  I used to have them write a note to their friend about looking for a job, and then write the note to grandma, and again to a prospective employer.  We then discussed the difference in the language used in each situation which also helped. If this is not what your question intended, answer one would be more helpful.