using field calculator in Qgis 1.8.0 to get the area, it shows just right at first but when I save it, the values become Null.
What Coordinate system is your shapefile layer in? If you have something in degrees, the area will be computed in square degrees (which does not make sense, and will be a very low value).
Note that the project CRS can be different, but has no effect on
If your layer CRS is in metres, try a greater value for width. 99999.99 m² may be too small for the areas. The formatting is only applied after saving, thats why you see the correct result at first.
just a guess but this is what I did wrong... you need to make the sure the columns properties are set correctly if you are creating a new column in the attribute table
when setting up the column you need to make sure you have the numbers right ie whole numbers or decimal (or text) and width and precision as well