There is a 'say thanks' tab on the left hand side of the member's name. The tab is in brackets and it is on the very bottom of the member's username.
For example, I'm looking at your post and I see, on the left hand side, your name, Crashed, then under that, Newbie, then one yellow box, the number of posts you've made, thanked ( this is how many times you personally have been thanked ) and finally the 'say thanks' tab. It's a live tab/link so all you have to do is give it one click and you're done; you've thanked the member.