This is sort of like a discussion from a month or so ago about how to sit while kilted. That advice was to sit down, then slide back into the chair. Most people who wear a jacket or coat and sit down usually lean forward a little to adjust the jacket/coat, then lean back against the back of the chair. If you've slid back to straighten the pleats of the kilt, then lean a bit forward to adjust your jacket/coat, you can very easily pull up the plaid so that it won't get bunched or "pin you down" to your chair. I've not written this very clearly, but I think you know what I mean.

I recently sewed up a half-plaid like Matt Newsome outlines, but instead of keepers/belt loops, I sewed a 4" wide hem all the way across the short end of my fabric. I then slid my belt through this "pocket" (which looks like the rod pocket on curtains), and fastened it around my middle. I gathered the fabric just the way Matt Newsome's pictures show, and follow the rest of his instructions in the above-mentioned article. The end results are identical. I intend to sew pleats into the belted end, but there's really not a need to, since the friction of the tartan against the rise of my kilt on my sides prevent it from sliding forward and enveloping me in tartan.