Try to fit the document squarely onto the glass of the scanner.
Some documents may have damaged edges, so you may need to turn the document and align it on a 'clean' edge. Once you have scanned the document you can rotate it to the correct orientation. Most image editing packages allow you to rotate in 90degree increments, use this rather than rotating by hand.
Inspect the image. If you see areas that are blurred, then it is possible that those areas of the original document were not touching the glass screen on the scanner. Use something which is flat and heavy enough to force the document against the scanner glass without breaking it.
You may find it is best to use a sheet of white paper between the heavy item and the document.
Where a document has thin pages, you may find that the print from the other side may show through when the image is scanned. To try and reduce this, use one or more pieces of white opaque copier paper between the back of the image and the lid of the scanner. This will help to reduce this.