The quality of a piece of writing is dependent on many things - but the use of a variety of sentence types adds enormously to the overall 'feel' of the text.
Try to begin your writing in a way that will catch your reader's attention. Short sentences achieve this most easily. Leave longer sentences for when they are really needed. Shorter sentences balanced with a few longer ones often create a snappier sounding, more effective style. Try to choose precise vocabulary rather than use lots of words to add meaning - this will keep your sentences clearer and crisper.