Is there an optimum enchantment strategy in Minecraft?

The mechanic in question here is the so-called Prior Work penalty.

Enchantable items, including books, have a Prior Work penalty stored in their NBT data (RepairCost). The prior work penalty is added to the base level cost of working the item in the anvil.

For a new item, this cost is 0, and it is independent of the enchantments on the item. Each time you do anything with the item in the anvil, such as repair, combine or rename¹, the prior work penalty is doubled, and 1 is added. E.g. it is 1 after 1 working, 3 after 2 workings, 7 after 3, or (generally) 2^N – 1 after N workings. After 6 workings, the prior work penalty of that item will be 63, making it impossible to work that item any longer outside of creative mode.

When combining two items, both penalties are added to the cost, but the new item's penalty is calculated using only the higher of the two penalties. The optimal strategy is therefore to start high up and combine items with similar prior work penalties.

For example, you want to get a Power V, Infinity I, Flame I, Punch II bow: From enchanting/loot/villager trading, you have a Power IV bow, and Power IV, Infinity I, Flame I and Punch II as separate books. All 5 items have a penalty of 0.

If you just go and put everything on the bow, the bow will have been worked 4 times. Instead, you start by combining the Bow and 1 Book (1 working), and the 3 other books into one (2 workings), then combine those two. You'll end up with only 3 workings this way. You could even combine the bow with yet another book (or bow) before that and still end up with 3 workings on the final bow.

^{1 Renaming does not apply prior work penalty in 1.9.}