Go through the Institutional Review Board if you're going to use students as test users, or if you want to publish your results.
To entice people to be test users, offer a small incentive. A $10 gift card to the local coffee shop is good. It should be enough to be a nice gesture of thanks, but not enough to be paying them for their time.
Promise to protect the test user's confidentiality and anonymity and keep the promise. Make sure the test user gives informed consent. Have it in writing and have them sign it.
Let the test user know that you want feedback, both positive and negative, and it will not hurt your feelings.
If you want the test user to narrate their thought processes and what they're doing as they go along, let them know.
Do not help the test user in any way, even if they get stuck. Getting stuck is a data point. If they are spending a lot of time without making any progress, you can suggest that they move on to the next task.
Use a script so that every test user has the same exact knowledge to start out with.