The best way a teacher can assist and help students with learning and communications disabilities is through adhering to each student's IEP (Individualized Education Program). Defined through the student's school social worker and special education teacher, the IEP lays out all modifications which must be made to insure the best learning environment for the student.
Typical modifications within the general education classroom are modified tests (multiple choice, elimination of long writing sections), extra time on assignments, having tests read aloud, private space for testing, not grading specific aspects of assignments (inversion of numbers or letters, spelling, grammar), and preferential seating.
It is always good to check with students with special needs regarding their understanding of concepts or ideas after introduction. Also, assignment books help the students with organization and objectives.
The most important thing a teacher can do to help a student with special needs to to listen to the student. How does he or she define his or her learning style (if possible)? What does the student identify as his or her strengths and weaknesses? With information like this, a teacher can insure small successes (something of the utmost importance).
As both a teacher and a parent of a very bright child with autism, I have sat on both sides of this question. One of the most important people in the IEP list is the parent who should be considered for every IEP written. I do understand that some parents want the moon for their child, but too many are not really listened to in the creation of an IEP. For example, my son's IEP had the standard "fill out his assignment book, pay attention in class, complete assignments on time even if modified" etc. When we had a meeting with the staff on his IEP, my objectives were "get out of bed using an alarm, take a shower, take the bus to school, meet his paraprofessional at the door, go to his first class and stay the entire hour" and then we could talk about filling in his assignment book. I wanted him to be in school even if it was difficult, even if the school had to call me to pick him up because he wanted to smash glass, even if I had to leave my classroom to take him home after another episode of no speech, no movement. My point is that teachers need to do all of the things literaturenerd has listed, but home should be consulted and listened to when creating the IEP. His IEP needed to state that anger and yelling were the worst things to motivate him frustrating as he was. Also needed was his pass to leave class which was frustrating to teachers as they had no say, but he might then be on the floor in that very classroom unable to move or speak. Yes, he was unusual, but extremely bright and very capable of learning, so he needed to be in a regular classroom. He was able to tell a teacher his strengths, what worked and what didn't, and his weakness with organization. Teachers need to talk to the student and the parents especially if things are not going well. Many parents can help if consulted because parents want their children to succeed and that means helping the teacher succeed as well.