Tie two pencils to each end of a ten foot string. Find the straightest putt possible on the practice green. Stick one pencil in the ground just behind the cup and stretch the other on-line with a straight in putt and stick it into the ground. Next, place the string about 3 inches off the ground. Place you putter under the string and use the top-line on your putter face to guide the putter blade through a straight line. The key to this drill is to visually keep the top line in-line with the string throughout the stroke.  This enables the putter to travel straight back and straight through the motion. You really cant practice too much with this drill as it ingrains the correct feel of a good consistent putting strike.