If you're PhD student, or colleague:
I am delighted to write letters of support for people whose papers I read and admire. Your papers enrich my life, and it's a pleasure to return the favour!
If you don't know who I am, and have never read my papers, the feeling is probably mutual. In this case, I'm not the right person to write a testimonial about your creative output, teaching, etc.
In either case...
It takes me a long time to write letters; so if you want a letter of reference or support, please ask at least 4 weeks in advance; If not, I will decline with the standard response "Other pressures, and your time constraints, preclude the crafting of a detailed reference letter, and compel me to decline; in no way should this response be interpreted as indicative of the worth of the candidate or his/her record". Make sure you provide with a statement of teaching, research, service etc for reference.
Also, I only write confidential letters, viz., confidential atleast to the extent required by the the University of California.
Please DON'T ASK ME to give you the (plaintext) letter to be sent elsewhere by you. I WILL DECLINE.