Martyn Lloyd-Jones, in his controversial book Joy Unspeakable illustrates what he means by baptism in the Holy Spirit with this story:
A man and his little child are walking down the road and they are walking hand in hand, and the child knows that he is the child of his father, and he knows that his father loves him, and he rejoices in that, and he is happy in it. There is no uncertainty about it all, but suddenly the father, moved by some impulse, takes hold of the child and picks him up, fondles him in his arms, kisses him, embraces him, showers his love upon him, and then he puts him down again and they go on walking together. That is it! The child knew before that his father loved him, and he knew that he was his child. But oh! the loving embrace, this extra outpouring of love, this unusual manifestation of it—that is the kind of thing. The Spirit bearing witness with our spirit that we are the children of God. This is the outstanding characteristic of being baptized in the Spirit. (Martyn Lloyd-Jones, Joy Unspeakable, 95-96)
Wether you agree theologically with this definition or not, you have to admit that as a Christian, you long for this to be true in your life.