I've used apostrophes in the past
The "bar" character seems logical
since (I think) it takes up the least space of any of the character...
In reality I typically revert to plain old
I also would avoid anything that might get used as script parts.
! $ *
If you import stuff into Excel, it's good to avoid any math operators
- + = * /
I think Word uses the tilde to denote temporary backups of open docs
but I think it is legal to use in Word doc file names.
Actually, though I'm not a programmer, I think the bar might get used to denote lists
item one | item two | item three
Which is too bad... I think they're kindof pretty.