First of all, after throwing your shuttle, ensure that the weft goes round the edge warp neatly. Pinch the warp and weft together and gently pull the weft into a 45degree angle.
If the weft is taken straight across the warps there is no extra weft to mould round the warps, resulting in the edge wefts being pulled in.
To test how much pull-in there will be, hold the weft at the edge of the selvedge and pull up into the 45 degree position and you will find out how many warp ends will be squeezed over both edges.
Being vigilant over this will ensure nice neat, even selvedges that don't pull in.