Firstly check for any stress cracks which could mean that the stove glass will eventually fail – probably at the most inconvenient time. If your glass is always particularly dirty and covered in brown stains then you should take steps to monitor the moisture content of your wood (you can buy a cost-effective moisture meter by clicking here) or how you operate the stove (too much slumber burning). When burning wood, and when the wood is fully burning, completely shut off the primary air (usually the bottom air control) and ensure that the secondary air (usually the top air control – but not always) is fully open and adjusted to the desired flame pattern.
Closing the secondary air down or severely restricting the amount of secondary air (no matter how much primary air you allow) will almost certainly reduce the effectiveness of the air wash system causing some of the sooting and staining from the non-combusted particulates and flue gas. If you suffer from this problem then we would also advise purchasing a flue thermometer which will indicate when you are burning your wood inefficiently and likely to be creating nuisance creosote. For more information on flue thermometers please click here.
Also check the condition of the glass gasket / seal which allows the stove glass to safely expand at a different rate than the metal door. Over time these become compressed and loose their resilience to the sprung glass retaining clips thus creating the potential to crack the expanding glass. If your stove is more than two years old or the seal looks in poor condition we would recommend replacing this seal as soon as practical even if the glass isn't broken. Failure of the seal may also be indicated by loose glass retaining clips (which should never be re-tightened on an old seal).
Please see the below video by Esse Stoves showing how to safely replace glass on their stoves.