Straight punch the sand 30 times and repeat with the backfist. Do 100 spearhand thrusts into the sand. You can also rub the sand between you hands to toughen the skin. Some iron palm practitioners feel that it is unwise to train the knuckles of the fist because of possible long term joint damage. This is fine for strict iron palm fighters, but if you train in any fist striking art, it may be wise to strengthen your knuckles.
Chinese acupressure teaches that training the fingertips can weaken the eyes.
Take this into consideration when training spearhand but also realize that plenty of karate stylists train fingertips and can see just fine. Different teachers advocate different numbers of strikes per session. Some use hundreds or even thousands of repetitions. Some say to train three times a day, others say you must train the exact same time everyday without missing a day. Maybe these routines are ideal, but with the method I outlined above, you can train whenever and even miss a day or two.