Ever since Art vs. Science I've been choosing based on which team is less popular and/or less likely to draw the worse players, and I make sure to pick the other one. I have yet to lose a Splatfest since. I really just play for the Snails, Splatfest feels mostly like a chore to me honestly. Grinding Turf War is just not enjoyable to me, so I want to make sure my path to max points is as quick and easy as possible.
Being on the losing team is just such a slog, between the long lobby times, minuscule number of points earned, and awful matchmaking. I'm always completely indifferent to the themes anyway, I never have a preference so I figure I might as well just go with whatever will get me the most snails with the least amount of difficulty.