If you've fundraised for a previous instance of the same event, you will be given an option at sign-up (in the detailed registration process) to import your contacts (and fundraising page welcome message!) to the new event instance.
Furthermore, even if you do not import your contacts upon sign-up, or you sign up through the quick registration process, you can access that option in your contact book:
campaign and imported your contacts on that campaign (learn more), you can import those contacts into any other campaign on which you are fundraising. This is especially helpful for year over year fundraising if you want to import your contact book from the previous year. Import your contacts in this way by navigating to your Contact Book in your Dashboard, then click Import Contacts. At the bottom of the resulting lightbox will be an option to import contacts from a different campaign, and you can even choose which campaign to import from.
Unfortunately, there's currently no way for fundraisers to import fundraising page customizations from a previous campaign.