Spelling shame

Today, I had a chiropodist's appointment. When I arrived I was asked to fill in a form with a few bits of basic personal information. No problem, until I came to 'allergies'. I have an allergy to a common antibiotic, but I can't for the life of me ever remember how to spell it! My pen hovered for a moment, then I plumped for "penecillin" ... Sat in the chiropodist's chair, the polite conversation inevitably turned to:

"So what do you do?"
"Erm, I'm a writer. I mostly write dictionaries."
"Oh really, dictionaries?! You must be very good at English."
"Well, no, not really. My spelling's terrible actually. I've probably spelt penicillin wrong on that form, haven't I?"
(Looks at the form) "Yes. ... Oh well, we're all human, aren't we?"

I feel as if I ought to be able to spell better, but there's just something about certain words that I have a mental block with. I've always been the same - at school I was always being nagged by teachers about my spelling. I think it's something to do with spelling being something you just have to learn - I've never been good at learning things by heart - I have to have some kind of logic or reasoning behind something. I can explain complex grammatical rules, but I can't remember whether it's an e or an i in the middle of words like penicillin!

Thankfully, I know which words I can't spell, so in most circumstances, I just check - I've got a pile of dictionaries on my desk, after all! I've even got my most frequent problem word on a little post-it note on my monitor ...