The easiest way for returning users to set up credit card processing on a new event site is to migrate (copy) the settings from a previous event site.
Returning subscribers can use the migrator tool to pick and choose which elements of a previous event site to replicate on your new site. If you migrate all options, you can effectively create a clone of your of past event - but without any transactions/orders/history. The previous event site remains intact.
If you've already setup your new site, you can still migrate the credit card settings - just be aware that migrations will not overwrite any existing credit card settings. If you have already worked on the form at Site Settings > Customize Your Site > Payments/Credit Card > Setup Credit Card Gateway, be sure to empty any field entries and set the gateway to "none".
Obviously, if you have changed processors since your last event, you'll need to enter the new credentials manually. If you aren't sure whether your gateway/merchant account is still active (and you're not sure who to contact), you can migrate your settings and run a live test transaction to verify.