I have long strong nails but am just lucky
Rubbing any nut oil into the nail is good for strength (almond oil)
I never paint them
I never file them, I round them off when i cut them and just leave them
Sally Hansen's nail biter - it's a miracle cure, tastes s**t so you really dont want to bite them and makes them stronger. I bit my nails for years and they were really weak. A friend recommended it to me last year and within weeks I had gorgeous long and strong nails....that never split.........invest it'll be the best �3.75 you've ever spent!!
Apart from eating a good diet, for beautiful nails, scrub them very gently every time you wash your hands - and always take the time to paint your nails carefully with a couple of coats of nail polish. It sounds odd, and I don't know why it works, but it does. I think it's probably because by doing this you're consciously taking care of your nails.
I have the weakest nails you could imagine and growing them is a nightmare. I have found that painting them does keep them stronger and encourage them to grow, so that's good advice there from other ABers.
I've also been using OPI Nail Envy as suggested by Pastafreak and found that it definitely helps if you stick with it. I also use OPI Avoplex oil, which helps keep the nail and cuticle hydrated. I have heard that a product called Seche Vite is supposed to be excellent, though I've never tried it myself....yet!
If you feel that your nails are too short or are in too bad condition to paint, stick to a clear nail polish until you are ready to show your nails off with colour.