If your outside selvedge threads begin to splay out as shown, there is too much angle in the diagonal of the weft. You must stop this or the warps will just continue to splay out more and more.
To correct this problem, throw the next weft, and while it is still loose in the shed, tug the previous weft at the selvedge, pulling out the tiny bit of excess weft.
Then, Take up that extra weft in the new shed and beat it in as usual. The tiny bit of slack that is taken out will straighten the warps. When the selvedges are back in place, decrease the angle of the diagonal in your wefts. You should need to make this adjustment only a few times to get the selvedges back in place. If you are throwing the shuttle on the shuttle race as I recommend, the way to decrease the angle is to move the fell of the cloth a little closer to the reed.
One common selvedge flaw is a characteristic ridge or ripple caused by a combination of too much weft in one shot, and too little weft in the following shot. The ridge is produced by changing the shed before the weft is beaten into place. With each shot the problem becomes worse.