If someone’s taken the time to write about you, responding to positive reviews so customers know that you appreciate their compliments. ESLwatch suggest responding publicly, you may well know the reviewer if you have ther contact details you can thank them in private. You may also have to decide if you answer every positive review or just the ones that stand out.
Do not offer tangible rewards (e.g. gift vouchers) to reviewers, because this can be construed as a bribe for future positive comments. Nor should you annoy reviewers by targeting them with advertising or asking them to join your mailing list. It's best to simply introduce yourself, stay brief and to the point and say thank you, use your judgement on a case-by-case basis.