Probably most people think they're good enough to do copywriting, and it looks pretty easy at first glance. It's probably the case that you've come across many sales letters in your IM career. Those letters don't look very hard, the words aren't very tough - and you're only trying to sell something. If you don't know this, copywriting is not the easiest thing to learn how to do well.
The solution for most online marketers is to try to find the best copywriter they can at the cheapest price. That's a good option for many who feel they aren't the best writer to hold a pen, or type a keyboard. But if you think you can give it a good shot, it's worth your time to learn the basics of it if you don't have the funds to outsource it.
A favorite for many writers is to weave a story in their copy. Most marketers make the mistake of only talking about their products in their copywriting. A story will help draw the reader into your sales letter or other copy. The reader must identify with your story, and it needs to be something in common with the product you're selling. Whether you're selling a product, service, or trying to get an optin, that is your most desired response; and that is the goal of your writing. If your story misses the mark, or you fail to make a connection, then it all falls apart. Your copy needs to make the reader feel a sense of urgency without making them feel pressured. Your campaign will be a dismal failure if readers get the slightest hint of "sales pressure." You can find great examples of this just from your own shopping experiences. Have you ever been totally turned-off from something because of sales pressure? But you most likely did buy from the person who offered no pressure, explained the product benefits, and who was just polite and friendly.
It's fine to break some rules if you do something good with it. You pretty much want to always spell properly, but good copywriting ignore most rules of grammar. The best course of action is to always get the facts, and it won't hurt you to educate yourself a little bit on this matter. Of course there can be a fine line, but not always - just try to use your best common sense about it. Becoming a good copywriter often takes years of work. Professional copywriters only make it look easy because of their level of skill. At the same time, for internet marketers, copywriting is a skill that must be learned. Knowing even the fundamentals of copywriting can really help you a lot in your business. When it comes down to it, you have to use copywriting to sell anything. If you don't know how to sell your products, why are you trying to sell them in the first place?