Do your Research
When looking into your own caterers make sure to shop around - gather quotes, check reviews and even meetup with potential caters and gather sample menus. Try to get your potential caterers to compete against one another - telling one company of a lower quote you have received could help you secure a better deal.
DIY as Much as You Can
Most caterers will add a mark-up to items so looking around at other external equipment hire companies could help you to save.