If left to their own devices, IVs are generated randomly. If you do not take steps to alter them, you have very, very, very low chances of improving, or even equating IVs from parents to children.
The most efficient way to improve IVs is to use the Destiny Knot. When "creating" a new baby Pokemon through breeding, 5 of the 6 Pokemon's IVs will be selected to be inherited from the parents, with the remaining IV being generated randomly as normal. For each of those 5 inherited IVs, that value will be chose between the Parent Pokemon at random, to be given to the baby. By selectively swapping out parent Pokemon, you can progressively improve the IV count of the parents as a pair, improving the chances of getting baby Pokemon with more and more IVs.
There is also a group of items called the Power items. Each one correlates to a specific stat, and when held by a parent Pokemon will force that parent's IV in the corresponding stat to be passed to the baby. Unfortunately, this method cannot be used with a Destiny Knot, and if both parents are holding different Power items, which one takes effect is a 50/50 chance. This also means you are leaving the other 5 IVs to be generated randomly as normal. Generally, this is only a valuable method for setting up a 0 Speed Pokemon, to be used for breeding a Trick Room team. Once you get a Pokemon with 0 Speed, you can go back to the Destiny Knot method, because the 0 IV can be passed down just as readily as a 31 IV.
TL;DR -- The Destiny Knot is an essential item for anyone attempting to breed for IVs and it is the only way to ensure you increase your IV count.
