Know autistic students' learning style (auditory, visual or by touch). Since autism causes children to become overwhelmed by too much information coming from the senses, you'll want to only focus on your students' specific learning style. For auditory learners, keep the visual aids at bay--and vice versa as your autistic students may not respond at all to speech, so try using flash cards or posters and keep spoken words to a minimum. If your students learn by touch, use the actual objects to teach (have him touch the objects while you explain the lesson).
Find a nice, quiet, visual place without too much stimuli to teach. Most students with autism will be bothered either by noises or visual objects such as light or movement. If you want your autistic students to pay attention, be sure to remove all bothersome stimuli from the room. Keep everything simple, and only have one lesson out at a time so the students do not become overwhelmed.
Use coursework specifically for autistic students, either given by the special education program at a school or found online. Thanks to all the autism foundations out there, you can find lesson plans made for autistic students on the Internet. If you can't find what you're looking for, talk to the special education program in the student's designated school and ask for materials.
To keep autistic students interested, use their fixation in your lessons. Often, autistic children will be fixated on particular objects such as seashells, spaceships or bananas. It could be anything. Instead of being bothered by this, use it in your lessons. If you're teaching subtraction in math by using objects, use the object of fixation (put out five spaceships and take two away, for example).
Remember that it's difficult for autistic students to generalize. People with autism learn very specifically and often struggle with concepts such as, an orange is one of many kinds of fruit. It's hard for people with autism to see the big picture. In your lessons, use lots of different objects or flash cards that all look different to help the students generalize. Otherwise, your students might think the only objects that can be subtracted are spaceships.
Use the Applied Behavior Analysis (ABA) method of teaching, or hire a tutor trained in ABA. Understand that ABA is a known successful technique to teach autistic students. It focuses on reinforcing positive behavior and attempting to get rid of negatives. Since autistic children have so many negative behaviors, you'll need to work on these and social skills as well in order to teach them.