Once you build a fence it is inevitable that the weather is going to take its toll on it. Fence posts that were originally cut square allow rainwater to soak right into the wood resulting in expansion and splitting of the wood. We've found that by cutting the fence posts at a slight angle the water drains right off rather than sitting and soaking into the wood.
A simple, easy way to measure postholes as you dig them is to mark 6-inch graduations up the handle of your posthole diggers (a permanent marker is ideal for the job). A similar approach works well for a shovel handle; only here you can measure from the handle end instead of the blade end so you can invert the tool to measure as you go.