True gardeners, it seems to me, aren't interested in "tips." They know that tips will usually fail them in the end because their own garden, its soil, its microclimate, its exposure, will exert an influence that no other person can guess at without first-hand experience.
Gardeners realize that a "green thumb" comes from deep knowledge and experience, and that "tips," for the most part, are quick explanations of routine mechanics.
A tip about pruning rhododendrons can tell you when to cut, but not necessarily the aesthetic aspects of what to cut. However, a gardener who truly understands his rhododendron and its needs can intuit when, and how much, pruning should take place.
Gardeners, in my opinion, simply want to know everything about plants. They want to know where they grow, how they look, what conditions they prefer. By inspecting a well-grown specimen, a gardener might deduce whether he would have similar success on his own property.
Gardening is a passion, a pursuit, a discipline, the same as throwing pottery or automobile mechanics. There are no magical thumbs involved, just hard work.
And for those who insist, here's the universal gardening tip: Dig a hole. Place plant in hole. Cover the roots with earth. Enjoy.