Retainer agreements (also referred to as representation agreements) are a type of compensation agreement with lawyers either for reserving their employment or as compensation for future services. Also inside the agreement are details on the scope and procedure for the representation.
A retainer agreement is a long-term work-for-hire contract between a company and a client that retains ongoing services from you (as a consulting business) and provides you with a stable amount of payments.
Here's my advice: Pitch only to existing clients or clients you know well. Never pitch a retainer agreement to someone you've never worked with before. Present the retainer as a way to get “front of the line” status. Highlight the benefit of predictable budgeting. Include a small discount (maybe)