Hi Jan
Like ken say's, it's easiest when still planing. The faster the board is moving, the easier it is to get onto the new tack and get going again.
FAST TACK:
*At full speed, back foot out of strap, leave front in strap, carve up wind but to much or you will lose speed.
*Rack sail towards the back of the board, this will keep you light footed on the nose. Keep sail at arms length all the way through the tack.
*Jump across to the new tack much earlier than normal, use two steps only. Both these things will help maintain speed.
*As soon as your on the new tack, get real low hanging of the boom to get mast foot pressure, kick the the board nose of the wind, sheet in, front foot into strap, pump, and your away.
This should all happen very quickly, quicker than saying "Tiki man"