CATCHING THERMALS
When you expect a thermal or you know there is one and hope to use it, fly as lightly as possible on the controls and feel for any perceivable sense of dynamic. Merely wandering can prove counterproductive, for it allows lift to push an undirected sailplane away and into sink.
Instead, try some form of logical search pattern that will keep you oriented so you can return to any particular point within the search area. Experience will help in choosing what sort of search pattern to employ. A very broad circle may suffice, but a square, rectangle, or figure eight might work better.
It's often best to try the highest ground you can reach first, even if it seems less promising than some lower spot. (If you flew over lower ground first and failed to find lift you might then be too low to try a higher place.) As altitude is lost in the search, that's when you should explore progressively lower areas. Of course you must plan any search pattern in mountainous country so that, if nothing is found, you'll still have sufficient altitude to glide out to a safe landing...