Got a guesser/inaccurate reader on your hands?

Here is one thing to try: The "Notched Card" Technique.

Developed by Tom and Hilse Burkard of the Promethean Trust, this technique has cured the guessing/inaccuracies of many a child. See the following link for a picture and description.

http://www.piperbooks.co.uk/documents/Notched_Card_Technique_000.pdf

The notch can reveal just one grapheme at a time, or (for those students who are not beginners) one word at a time.

Here is a discussion about the notched card:
http://rrf.org.uk/messageforum/viewtopic.php?t=3815&highlight=notched+card