The first and possibly the most important is an adequate bowel preparation
The ability to see pre-cancer polyps and colon tumors is dependent on you having colon clear of residual stool. This is dependent on you completing the bowel preparation as prescribed as well as drinking lots of supplemental fluid during the prep. All preps require you to fast from solids for a full day before your exam and only drink clear fluids. If you have had a prior poor prep or have tendency to be constipated, take medications or have conditions that slow the movement of your bowels then you may be asked to stop eating solids two days before your exam.