With larger key accounts you should aim to become their trusted advisor.

How do you do that?

Time.

Nothing will help you be more successful than spending time with them and provide meaningful value.

For example: 

• Look for ways to be of help. Sometimes this might be non-monetary value such as a workshop or briefing. 
• Learn and adapt to the organisation’s culture and terminology. They will be much more comfortable around you when they know you ‘get’ them!

As you do this you will gradually become the default choice among your competitors.  

Over time you become their trusted advisor.