How to maintain straight selvedges is the number two pain point for weavers. It comes second only to warping. There are three things you must do if you want to maintain tidy edges and prevent draw in.
The weft yarn doesn’t travel in a straight line from selvedge to selvedge. It has to bend over and under the warp yarns. To allow enough length for the weft to do its thing, you need to lay it in at an angle before pressing it into place. How much of an angle depends on the width of the fabric and structure.
Your angle is too low if your selvedge begins to draw in, crowding the other threads.
The angle is too steep if loops appear at the edges.
Advance the warp after weaving 2-3 inches of cloth. The closer the cloth is to the rigid heddle, the greater the tension placed on the selvedge threads. Notice here how on an open shed, the selvedge threads that are closest to the rigid heddle are being pulled apart.
Weighting your selvedges gives them an extra bit of tension making it easier to maintain crisp edges. My favorite way to do this is to slip an S-hook around each selvedge and let it hang off the back beam. This is also a nifty trick if you have a loose thread in the middle of your warp.
Threading selvedges in a hole will also offer a tighter tension at the edges than threading them in a slot, but not all patterns call for an odd number of warp ends.
Helping weavers past their pain points is what I do. If you have weaving questions, hop on over to Yarnworker’s Ravelry group and post your questions. I’ll bet there is a weaver out there who has the same question you do.
P S. Life After Warping: Weaving Well on a Rigid Heddle Loom and Weaving Made Easy are full of these kinds of tips.