Here's what I have in common with Papa Hemingway: I love words, and I'm a writer.
When it comes to writing ads (and when I say ads, I mean everything from web pages to emails to postcards!), it's hard to find better advice than Ernest Hemingway's Four Rules of Writing.
Hemingway's Four Rules of Writing
1. Use short sentences.
2. Use short first paragraphs.
3. Use vigorous English.
4. Be positive, not negative.
Those rules are posted on the wall behind my computer monitor. You could do worse than to post them near yours.