How many guests will be attending?
In the early stages you will need to finalize the guest numbers a s early as possible. The sooner you know the exact amount the easier it will be to organize the things, and that's not just the catering. It will be much needed information for the venue size, table arrangements and parking space if required.
What style of event and food will you be putting on?
The happy couple to be know what food they would like before they contact any caterers then the whole process will be more straight forward and quicker from start to finish. The catering company will also be able to give you a more accurate quote first time round rather than relaying questions and ideas about menus and timings etc. That said if you need help and advice compiling your menu then any good catering company should be able to provide this.
Saving money on dessert.
A good way to save some money is by using your Wedding cake as the dessert. This is quite common practice and is totally acceptable. Guests will only be trying to eat it on top of an already full stomach any way.
Book your catering company early.
When you know a bit more about what you want then it is time to call round a few catering companies. I would suggest trying to stay with caterers local to your venue. This can have advantages such as the caterer may be familiar with the venue, and be able to source better local foods.