Be sure the reader knows who is speaking about whom. Read very slowly and aloud. Be sure to check your quotation, then refer back to the original. Do not rely on a quotation to get your point across. Always provide your insight.
Quotations provide support and color.
Try to hit a balance between quotation and your own commentary. A string of quotations is not considered your work. After mentioning a source's full name, it is then desirable to use the last name only from then on. Use quotations to liven up your writing. Choose them carefully.