The sample (DNA template) purity and quantity are the most important factors affecting the success of sequencing results. Most common Sanger sequencing templates could be a plasmid, PCR products or DNA made from Rolling Cycle Amplification (RCA).
As for purity, the template DNA should not be contaminated with proteins, polysaccharides, RNAs, or genomic DNA. As for the quantity, a proper DNA quantification method other than a mere spectrometer for 260 nm and 280 nm measurement should be used to measure the concentration of the template DNA. The right amount of template based on the length of the template should be used in the sequencing reaction.