Have a Backup Payment Plan
If you're planning to pre-order you'll want to have a backup payment option ready in case the card you use to pre-order your device gets flagged by your bank or credit card company.
If you don't normally make purchases at Apple or if you usually don't make big purchases on the card you're using to buy the iPhone Xs, your credit card company might flag the purchase. This could hold up your pre-order.
This is an issue that pops up every single time Apple releases a new iPhone and we've dealt with it ourselves in the past. If you aren't prepared for it, it could cause a delay and prevent you from getting your new iPhone on time.
If you have another credit card, you'll want to have it on hand when you go to pre-order your iPhone Xs or iPhone Xs Max.