The obvious answer is stay indoors during a thunder and lightning storm. But, if caught outdoors don't shelter under a single tree in a large open area. Or under the tallest tree in a forest. If on the summit of a bare hilltop, kneel down (in a trench, hollow or drain, if possible) and cover your head with your arms and keep your head close to the ground (this advice is from a survival book). When, several years ago, I was caught is a severe thunder and lightning storm, while wild camping on very high ground in North Wales, I simply lay as flat in my tent and kept my fingers crossed as lightning flashed for several minutes (it seemed longer!) around me. One thing to avoid is to carry on playing Golf!