First, if your toothpaste has glycerin in it, stop using it. If your teeth are as bad as you say, I wouldn't even try baking soda, since it's a little abrasive too. Go to http://www.toothsoap.com
and get some liquid toothsoap or tooth soap shards and brush with that. Look at your diet and eliminate anything in it that is acidic or that would tend to leach out minerals from your teeth: strong black tea, coffee, lemon, pepsi, coke, all sodas, tomato juice, etc...

Next, supply your body with the minerals it needs to rebuild your enamel. Either pick up a really good chelated multimineral supplement and take that religiously,break the doses up to 2-4 times a day, since the body can only absorb so much minerals at a time. Or start drinking smoothies nutrient and mineral dense green powders or fresh wheat grass. There are herbs that are mineral dense that you can also drink as a tea instead of coffee or black tea. Horsetail, I think is one. Also, you could find a recipe for a good bone broth (use organic if you can) and make that a diet staple. It would provide your teeth with the minerals it needs in a easily assimilated form too.

It is entirely possible to re-mineralize teeth. My sister repaired a cracked tooth by using toothsoap and a coral calcium supplement.

When your body is acidic and lacking in basic minerals that the major organs and muscles needs, it will mine it from other sources like the bones and teeth. If you can correct your acidic state and provide your teeth with the minerals it needs, your body will know what to do.

iolite

I have similar problems, my teeth being very yellow and also getting smaller. I like lolite's suggestions and may use some of them myself. I want to add that you may be grinding your teeth in your sleep, which definitely causes them to get smaller(grind them down) and also a dentist told me that it causes loss of enamel and yellowing.

A good tool for night grinding is a night guard. They can be bought for $20 or less or for as much as $500 if you get them fitted to you at a dentist. Also you might get some of Uny's tinctures for nerves, anxiety and insomnia. Even having parasites can cause night grinding, so doing some general cleansing and healing such as taught at this forum would be a good idea.

You might be brushing them too much and overbrushing removes enamel.