Preparation is key for this trail.
Water: You will need a water carrying capacity of at least 8-10 liters, more if you drink much or if the temperatures are high. In addition, bring a water filter and something to pre-filter your water with (a bandana worked well for me). I would not recommend a SteriPen here, as some water sources are very muddy, which will hamper the working of the UV light. Download and use the water report for information on water sources.
SOS: If you will ever bring a personal locator beacon (PLB) or satellite tracker anywhere, this is the trail to do it on. There will be no one to help you should there be an accident. Be prepared to have to wait days for rescue, even after an SOS call. Also, take into account that you will not have any reception, even for satellites, when you are down in a slot canyon, which is a regular occurrence on the HDT.
First Aid: Bring adequate first aid supplies and know how to use them.
Navigation: Physical maps, compass, GPX tracks. Make sure you have more than one way of navigating. I did most of the trail using a GPX track on my phone, but kept physical maps as a backup and made sure I always knew where I was on these maps. Download all necessary information and maps onto your phone before you leave, as you will not have cell phone reception for most of the way.
Rope: Bring a rope (paracord) to lower your pack down rocks or haul it up when needed. There are several sections where you won’t be able to climb up or down with your pack on your back.