I actually use the Vlisp help file.
No shame in that. If the help file wasn't there, I'd be completely lost. It's a pain remembering all that random syntax.
Me too - there's always a function that you don't use too regularly that requires looking up the syntax.
With all the talk about Editors in this thread, I still hold that the VLIDE is by far the best editor for AutoLISP - its got all the syntax highlighting, parenthesis matching, indentation and word completion but also has debugging facilities and demand-loading and a direct link to the help documentation - all of which makes everything much quicker.
Notepad++ is also a great free editor, I use it for writing the code for my site. I would note that although it has LISP as one of its supported languages, this is Common LISP and hence won't highlight those functions specific to AutoLISP. Of course, that doesn't stop you adding the AutoLISP syntax to the language file...