Unless absolutely necessary, never use "I" or "me." Too many instances will come off as poorly written.
If an opinion needs to be expressed, perhaps pose a question or present the opinion in the form of sarcasm or wit. This will follow Step 1, while still getting the same point across.
Never assume all readers agree with you, so the word "we" should not appear unless referring to actions or opinions of a previously described group of people.
Use of possessive language such as "my" or "mine" can imply false notions of ownership.
Be descriptive. The easiest way to avoid informal language is to focus on painting beautiful images with the words.
Stay consistent. Do not flip flop between imagery and opinions. This will only confuse readers.
Proofread like crazy! Throughout the course of writing you are bound to slip up and offer informal language when better methods exist.