Never ever give up!
It only costs a stamp, an envelope, a piece of paper, and some toner. No biggie. As mentioned, tweak your GWs. Write different drafts. Shorten some. Send to varying addresses if possible. And look for contact names to send it to like CEOs, VPs, etc.
I just sent GW #26 to CapOne since September for two lates in 2002 (30 & 90 days). Probably not hurting all that much, but I'll never give up.