Hi Bob
Is it black hat to encourage +1s in return for a voucher? These are Google's thoughts on what a +1 is for:
+1 is how you signal your appreciation for anything that grabs your attention on Google+ or on your favorite websites. When you read a post that makes you want to cheer, +1 is your applause; when you watch a video that has you in stitches, +1 is your laughter; when you see a photo that perfectly captures that special moment, +1 is your 5-star review.
I guess on that basis that technique is not exactly white hat but perhaps more grey hat!
To answer your question re white hat tips, it is producing anything that fits Google's definition of a +1. I remember 10 years or so now Seth Godin wrote a book called The Purple Cow. The story is that when you see a field of cows you might mention it as you drive past the cows are soon forgotten. But as you drove by if you saw in the middle there was stood a purple cow, you would stop your car, jump out take a photo or a video to tell your friends and the world about it (also via likes, tweets and +1s) because a purple cow is remark-able!
So I'm not sure there are any tips other than that. It's finding the purple cow in your business, in the products or services you sell, in the way you treat your customers or the great information you provide on your blog. If you do then people will want to "remark" about it and tell their friends and the world!
So I don't think there is any secret sauce. Just be remarkable!
I hope that helps,
Peter