Now, we are going to talk about what you should (and shouldn't do) in the course of the appeal:
First, the insurance company or fiduciary has an obligation to provide you with a copy of their file, and all documents that relate to the case. 29 CFR § 2560-503.1. If the claim involves health or disability benefits, you are entitled to (1) all the documents the insurer relied on, and (2) documents setting out their claims handling policies. Make sure you ask for them.
Second, the Plan Administrator (i.e. the employer), is obligated to provide you with copies of documents explaining your rights, upon receipt of a written request. 29 U.S.C. § 1024(b)(4). Ask for all "plan documents" and also ask for the most recent copy of the "Summary Plan Description", which is a summarized version of your rights under the Plan.
Third, be mindful of time limitations. If you need more time ask for it, but be sure that you make your submissions within the time limitations given by the insurer. Otherwise, you could lose your chance to submit the additional evidence.
Fourth, gather up evidence that you will want the insurer (or, later, the judge) to look at in deciding your claim. Here are some examples:
You certainly want to write out a letter (or perhaps a Declaration) explaining why you are entitled to benefits. If you have a disability claim, tell the insurance company (in detail) how difficult your job is, and what about your condition prevents you from doing it. Be descriptive, but don’t exaggerate. If, for example, you have been to five doctors because of your condition, say so, but don’t say you have been to a dozen doctors. The insurance company may well get your medical records showing your doctor visits, and it will use any inaccuracies to show that you "aren’t credible."
If the case involves medical issues (i.e. health or disability benefits), make sure that you have evidence of this. Get your medical records and provide them to the insurer. And explain your problem to your physician, and ask him or her to write a letter. Provide the physician with enough detail about your condition and your claim so that he or she can write a good, detailed letter. Follow through, and make sure the physician actually writes the letter. Remember, this is your last change (probably) to get the physician’s opinions before the Judge.
If you can get statements from friends and co-workers addressing the difficulties of your job, and how your medical condition has impacted your ability to work, great. Remember, you won’t be able to call them as witnesses at trial, you have to get their testimony now.
Be creative. I once saw a claimant who had a difficult physical disability, who sent in a video-tape of himself. If you sent it in, the insurance company has to look at it, and it becomes part of the "Administrative Record" that you can use in Court.
Fifth (and most important), make sure you respond to the insurance company's reasons for denial. These are supposed to be set out in the denial letter; make sure the evidence you submit addresses this head on. For example, if the disability denial letter says you can do your job with your back condition because you only have to lift 20 pounds, make sure that you let the insurer know how much your job really requires you to lift. If they rely on an in-house doctor's opinion, get your physician to rebut it. And don't just argue with the insurance company, make sure you have evidence showing why the reasons for the denial were wrong.
Make sure you save copies of everything you get from the insurance company, and everything you send to it. Good Luck!