Stay low, and stay loose. Number one cause of most handling problems is too tight on the handlebars.
+1 A couple of years ago my brother and I were coming back from the Catskill mountains in autumn, both on KLRs, and he complained over the Comms about how the wind over the bridges was making him nervous.
He had a death grip on the bars and was really holding on tight with his knees and body.
I showed him that I was relaxed, that I'd let the wind push the bike, and use my relaxed body to flick it right back into my lane position.
Gusting wind is the worst, but you can see it coming, at the end of a treeline, the crossing of a river or Lake via bridge.
If it's a constant steady crosswind I shift my weight onto the side of the bike closest to the wind, and ride with a bit of a lean. It feels natural after a while.
The absolute worst was riding an overloaded KLR down the west side of Newfoundland from St. Anthony to Gros Morne as the wind was well up over 80kph, with a 12 foot rocky embankment on the right, and of course the yellow line. It felt like the front end was going to tuck, and the gusts had me all across the road trying to correct. It was nerve wracking and tiring at the same time. It stands out as the worst ride I'd ever care not to repeat.