Here's my tip - a Medical Journal for my kids. The idea originally came from my mom almost 11 years ago when JJ was born. She suggested I get a small notebook and write in it every time I take them to the doctor. I didn't take the advice until Tuffer was born. I wish I had used this notebook when JJ and K were babies. It has been so valuable. Since we move a lot, I have a detailed account of what past doctors have said, medication dosages, frequency of illnesses, etc. It was very handy when K had frequent bladder infections. I was able to tell them how many times in the previous six months they had occurred and what different types of treatments and tests we had done. It has been wonderful in keeping track of JJ's medication evaluations and also Tuffer's growth. It's just a simple 5x7" notebook with a pocket in front where I keep the shot records (also handy to bring along to doctor appointments when you move a lot).
I divided the notebook into four sections (positive thinking that we might have a fourth child someday) by putting a round sticker on the top right edge of a page. If you have more than four kids you might want to divide the books and have one for boys and one for girls. This has been the handiest thing ever. Every entry has the date, the reason for the appointment, weight and height of the child, diagnosis, tests performed, treatment given, and anything else I feel is important to write down.