Ask for copies of your medical records
Getting copies of your records is an important first step. Getting the records quickly will make sure that they don't get misplaced or lost. If you have gotten poor care, you are likely looking for a new doctor anyway and you'll need your records to show your new doctor. Make and keep a copy for yourself. Make sure that you get a copy of your entire record-- even pages that you don't understand.
Get a second opinion
Even a successful medical malpractice lawsuit won't make you healthy. If you're worried that you are getting or have gotten negligent care, go see another doctor. When you go, make it clear that the reason you are there is to get treatment. Do not discuss your potential lawsuit or try to persuade the doctor to be an expert. If the doctor has the sense that you have an agenda beyond getting good treatment, he or she is less likely to want to care for you.
Keep a journal about what is or has happened
The good news is that your memory of painful or unpleasant things usually fades over time. It takes a long time for a medical malpractice suit to develop and come to trial and you will be grateful later if you take the time to keep a record of what is happening when things aren't going well. Be careful not to exaggerate-- just tell it like it is.
Be very careful about complaining about one doctor to another
Even if they don't practice together, all physicians share some level of discomfort with the way the legal system deals with medical negligence issues. Complaining about one doctor to another (even if you have known the doctor all your life) runs the risk that you will be treated differently after the complaint than you would have been before. Talk to your doctors about medical things; talk to you lawyers about legal things. Don't risk a good therapeutic relationship because you need to vent.
Consult with a lawyer who does medical negligence cases right away
Medical malpractice cases are hard even in the best of circumstances. The longer the time between the treatment and the initial contact with a lawyer, the greater the chance that records have gone missing, memories have dimmed, and the trail has gone cold. It usually doesn't cost anything to consult with a med mal lawyer, and a prompt start makes it more likely that you'll gather the information that you need to make your case.