It is important to have the material finely ground for best results. Not grinding finely enough is a classic LB rookie mistake.
Your ground material should at least pass without pressure/force through the mesh of a common window screen. If it is this fine or finer, then that is a good start. Magic-Flight's standard "calibration test load" has an average "grain size" of 30 thousandths of an inch -- it's fairly fine, but not powder or kief. That would be a 200 Mesh filter, or approximately 75 microns particulate. Perhaps similar to an average grain of table salt and a little more. Clean picked material is processed using a conventional grinder and then reprocessed using an electric grinder before grading -- this provides a consistent standard to test with. Most users would not need to go through this much effort.
Start with dry clean herb (no stems or seeds) and use a good grinder. There are as many opinions about grinders as there are about vaporizers. In general, however, it's agreed that a two-piece yields finer grind than a four-piece. This is because the herb stays in the grinding area in a two-piece but falls through to a collecting chamber in a four-piece. The suggested method to get finer grind from a four-piece is to invert it for a few turns. This can be messy if you're using the kief catcher.
Electric ones work especially well, particularly on dry material; however, it is easy to overgrind and some electrics can be difficult to keep clean -- or more importantly to some, it can be hard to get back everything that went into one.
If your load isn't ground finely enough, it won't heat evenly and your vapor yield is reduced. Many users remove the load after a couple of draws, when the material is thoroughly dry, and crumble it with their fingers before re-loading. This exposes more area to vaporization and helps ensure even heating.
If in doubt, or concerned especially with efficiency, using smaller amounts can be helpful in some situations.