Now that you have a contact who has delivered results on at least one task, you should figure out their commitment level and domain of expertise. Here are some general levels from least to most commitment:
Level 0: The user will field the occasional question, but will not perform any actions on your behalf.
Level 1: The user will field questions, and is willing to test patches, beta releases, and other code changes to verify fixes and produce extra debugging information.
Level 2: Everything from Level 1, plus is willing to spend regular time working through issues with you. They will set aside time (maybe averaging an hour every 1-2 weeks) to actually work with you over the phone or IM (or even email if necessary) to collect information in real time.
Level 3: Everything from Level 2, plus will continue to assist you with some task autonomously. They will accept some level of tasking from you to perform during their daily work and will report back results to you. There are very few users that are willing to do this out of the goodness of their heart, or for love of system. Generally you will get a Level 3 user when some higher-up compels an employee to assist you for the good of the project.
In some cases, it will be immediately clear what level a particular user is willing to work at just based on what they offer to do. In other cases, a user will progress to higher levels over time simply because they feel rewarded for the work they do.
Beyond determining the level, you will want to get a mix of contacts from various departments or teams to get a wider set of perspectives and testers. For example, if you are making a billing system, you will want a contact from the sales team, the accountants, and any other group that might have it's own perspective. In general, you shouldn't turn away an additional contact from a group from which you already have a contact, but it makes sense to actively look for users from groups for which you don't currently have a good contact.
Tomorrow: The Distance Bug Investigation, Part I