In my experience, many teachers make poor use of teaching assistants, regarding them as a sort of junior helper on the same level as a school pupil doing a holiday job. This is unfortunate. The teaching assistant can be a vital component in the quest to raise standards. Here are my thoughts about how this can be brought about.

Outside of the classroom

The work starts before even setting foot in the classroom. Generally speaking, to be most effective in the classroom, the teaching assistant ought to be involved in all stages of planning, and given the opportunity to have appropriate training. In particular, the teaching assistant should:

Be familiar with the scheme of work being followed. She may even have had a hand in designing it or adapting it to the school's needs.
Understand where the lesson fits into the scheme of work, eg it may be an introduction to internet search techniques, which are to be further developed later in the course -- perhaps next year.
Understand where the less fits into the current unit of work, ie what came before, and what follows. In other words, the assistant needs to know that in order to understand the main point of this lesson, the pupils need to understand X, which was covered last week, and that if they master this week's objectives they will be in a strong position to understand the objectives of next week's lesson.
Understand the intended learning outcomes of the lesson.
Understand how the ICT can help to achieve those learning outcome.
Be familiar with the hardware and software applications involved.
Be familiar with the individual children’s needs.
Have access to the school’s data management system (as it relates to pupils) and be familiar with how to use it.

In the classroom

The teaching assistant given the job of supporting children with special educational needs should:

Set up specialist equipment if any is needed, before the pupils enter the classroom.
Focus only on the children with special educational needs, generally speaking.
Not do the child’s work for her.
Help children, where appropriate, by adjusting the computer environment. For example, use the display options (if you have access to them) to make the contrast better on the screen, or use the accessibility features if you are using Windows (look in the Programs-Accessories menu).
Have a range of paper-based resources handy, in case the computer system goes down.