Commonly overlooked whetstone flattening tools are an essential part of any sharpening kit. Once a whetstone becomes untrue it will not sharpen with the same level of accuracy and efficiency. Additionally, whetstones can become glazed with small metal particles which will slow the sharpening process and reduce the overall edge quality.
Ceramic flattening stones are a good option but can over time become untrue themselves. Although more expensive diamond lapping plates are hard to beat. Diamond plates flatten faster, last longer and will remain flat. Finally nagura can be used to clean and resurface whetstones.