Use Two Devices
If you're in desperate need of a new phone and you want your iPhone Xs ASAP, it might be a good idea to fire up the pre-order process on a few devices.
If you can, have your phone and another device handy once we get close to the start of pre-orders. That device might be another phone, an iPad, an MacBook, whatever you've got on hand.
Having another device open and ready could come in handy if something goes wrong during the pre-order process.
You might open up Apple's website/Apple Store app on both devices or maybe you have one set to Apple's website with another set to your carrier's website just to be safe.
It also might be a good idea to have one device connected to your Wi-Fi network and another connected to your wireless carrier just in case.