Like OnlyShallow and others have said, start from the bottom and be patient. Put a TV show on, get a fine comb, and move upwards in tiny increments, untangling all the lower bits first. It takes a long time, but that way you'll do the least damage to the hair you have.
If you come across a knot you can't pull your comb through, try to tease out the lower part of it first. If nothing is working for a big tangle, try to gradually pull little bits of hair loose at a time, removing as much as possible from the knot, and if you still can't get the core of it out you'll just have to cut it off.
As someone who's had waist-long hair for several years now, here are some other tips for maintaining it:
- If you're going to be doing outdoor activities, TIE IT UP. Always. Better yet, braid it. If I don't tie my hair into a tight plait when I go snowboarding, I get giant clumps of horrendously matted hair at the back of my neck. I also braid my hair every night before bed to avoid tangles.
- Avoid shampoo as much as possible. Shampoo strips your hair of natural oils that protect it. Yes, this is what shampoo is supposed to do to stop your hair from looking oily - but if you use shampoo every day, your scalp will overcompensate by producing extra oils. Try spreading out shampoo use as much as possible. I only use shampoo on my hair once a week.
- Never shampoo the ends. When you do use shampoo, just rub a small palmful all over your scalp to clean the roots. As you rinse it out, enough will wash through the rest of your hair to remove most of the grease. You should never need to use shampoo on the lower part of your hair.
- Regularly use conditioner on the ends. You don't need to condition your scalp. After shampooing my scalp as described above, I use a big glob of conditioner on the bottom six inches of my hair. I leave it in for three or four minutes before I rinse it out.
- Get your hair trimmed a couple of times a year. No matter how careful you are, some split ends are unavoidable, and split ends will split further and further up your hair if you leave them as they are. Trimming the split ends off every six months or so will actually make it easier to grow your hair longer.
I use both head and shoulders as well as conditioner once every 2 days. Is that doing it wrong? I don't like to shower too often (unless I feel dirty or sweaty) as my skin gets dry.