Especially working in Preschool rooms, students with multiple disabilities and with children with more profound autism, I have run into some kiddos with pretty severe feeding issues. Rule number one is: don't go right for the mouth. There are often emotional and sensory issues tied in with feeding issues. Many children have had bad experiences with others in this area. If you are going to try putting your hands near, on or even in the child's mouth (which is often the thing to do!), start by establishing a comfort level with touch at the extremities. Start by interacting with the child's hands first. Use firm, steady touch and include some gentle squeezes or pressure. Start working your way up an arm. As soon as you see pulling away, back off. If you can get to the point of touching the child's face, you can use the desensitization technique, below. You may do better using something like a washcloth or a chewy item to encourage firm biting and firm touch around the mouth (playing tug-o-war with a washcloth in a mouth can look odd, but can be very therapeutic). It may take several sessions before the child will accept you touching near or on his or her face. Of course, if there is wasting or failure-to-thrive issues you may not have this luxury, but if you do, take your time!
Yes, that whole paragraph is all rule number one. :)

One other quick tip: if you are trying to encourage tasting new foods, try putting them in a little piece of cheesecloth to chew and taste. If the child hates it, he or she can get rid of it quickly without bits of it staying in the mouth. Less scary!