When I set up my rate sheet in 2003, I expected that my normal mode of work would be retainers. I also figured that time and materials or hard-dollar jobs would take 60-90 days to get paid. Consequently, I offered a 10% discount for a 350 hour retainer and a 20% discount for a 700 hour retainer.
It turns out that Supply Chain Management guys HATE retainers. I mean REALLY HATE retainers. Eight years ago, SCM was not as powerful as it is today and a couple of companies took advantage of the 20% retainer. It really worked out well for everyone--I paid myself from the banked retainer and my client saved 20%. The last MSA I negotiated, the SCM guys made me print a copy of my rate sheet without the retainer option included. I just redacted the retainer portion and everyone who sees it asks "what was cut out????". I tell them it is discount language, nothing they need to be concerned about.
Retainers were about 8% of my total billing through 1/1/2011. That number will just decline from here. I'm not sorry that I included the option, but I like not having to give the discount. It is the only time I ever discount my hourly rate. If someone asks for a hard-dollar bid then I double my hourly rate and add 15% to the inflated total because I don't like taking the risk myself.
David