Sounds simple doesn't it. Well there are a few basics that if remembered will make yours and others searching for email easier in the long run.
First, don't hijack a conversation thread. Yes, you know who you are. Rather than click the compose button you find a thread started by the person you want to email and use the reply function. Why is this bad? Well if you want your email found later if will be difficult to find under an unrelated topic. In addition, if a co-worker is following the thread or searching for it later a whole bunch of un-related messages come up.
Its really quite simple to compose an email. Click the big compose button and you are set. Simply begin typing someones name and let Google do its magic. Don't forget the subject, yes its important. It makes it much easier to find your messages later and re-assures your co-workers its not a virus. Think of the subject as your chance to add keywords or labels to your email.