Ask HN: How do I write a software license?

I need to choose (or write) a license for a piece of commercial software. I wonder if anyone here have some experience, ressources or good advice?

Are there some commmon boilerplate licenses I can find and copy, or do I need to write one from scratch? Can I write one myself, or do I need a lawyer to do it? Can I just write something in plain language, or do it have to be WITH CAPITAL LETTERS AND IN UNREADABLE LEGALESE to be valid?

The conditions I need are fairly ordinary - the software is licensed per user, but I want it to be freely usable for tryout and educational purposes.