(You can put this in ~/.sbclrc or your implementation's equivalent dot file!)
(Tangentially: I've encountered Common Lisp libraries in that wild that actually break if you do this, highly surprising bugs where the author was doing something too-clever with symbols and compared their representation against hard-coded strings, such that changing your *IDE's default print case* had the effect of breaking a very low-level library. This isn't important to anyone else here; I just wanted to vent. I have so much programming pain that only I know about and no one else in the world shares).
CL is case-sensitive, but it handles things so you typically don't have to be aware of it. I don't think I'm smart enough to try to do something clever on top of that, so I hope I can avoid your pain.
I found a snippet somewhere that messes with string interning and so symbols don't instantly get capitalized, but there's still way too much shouting.