Efficiency 5 is no longer obtainable in stone or diamond tools through enchanting. If it would still be obtainable though, there wouldn't be a trick on getting it, just a very small % that allows you to get it (luck basically).
In 1.4 it is possible to get tier V tools using the anvil. They will no longer be unobtainable which is why everybody is in a frantic rush to sell their tool at the moment.