It depends on your target demographic.
1) First, put together a plan.
2) Second, identify the right people within your target demographic. The ideal beta tester is an innovator or early adopter, in Crossing the Chasm parlance, because of their propensity for providing feedback.
This is in contrast to the early or late majority of your market, who may not necessarily provide useful feedback for an early stage product.
3) Once you've identified these types of people in your market, the next step is to find out how to get their attention. Where do they congregate? What kinds of media do they consume? Just to throw out some ideas (though this will vary significantly depending on your target demographic):
And for the people you recruit as beta testers, ask them if they can refer others as well.
4) Next, craft your message. Again, the kind of message depends on your target demographic. But you should at least include:
If you've found true innovators and early adopters, you may not need to give them any gifts as incentive, though it's something to consider. If you will be offering a paid service, you can offer them a free month or year too.
5) Finally, make sure you are responsive to your beta testers to keep them engaged. Make sure they feel heard and that their feedback is being taken seriously. Be respectful of their time, while maintaining some kind of regular contact during the duration of the beta test period.
If you have specific goals for your beta test, consider creating a survey that your beta testers can fill out. This way, you get to cover the specific areas in which you want feedback. Include an open-ended field for additional feedback as well.
Expect to have some drop-off. Some people just get busy over time and cannot participate any more. But if you can keep a decent number of them, you are doing well.
And most of all, keep an open mind! You may learn some interesting things that may run counter to your original assumptions.
Go where your target audience is. If you don't know that, then you're not ready to beta test, because you haven't done any problem/customer validation.