gistfile1.txt

BlockState bs = signBlock.getState(); // get state of block
Sign s = (Sign)bs;  // we know it's a sign already, cast the state to a signs State (org.bukkit.block.Sign)

s.setLine(0,"Hello World");//set first line

byte b = s.getRawData(); //get the byte of data this block holds (sign direction);
b += 1; // increment it (rotate sign slightly)
b = b & 0xf; //Keep lower 4 bits only (0-16)

s.setRawData(b);

s.update();//update block