Always try to have your whip in the outside hand when coming to a corner or turning question. If it’s a double of opposite-facing corners, have it in the hand to which your horse is more likely to wobble – they usually have a favourite side.
Sometimes just lightly touching the whip down the shoulder can be enough to remind the horse that it must stay straight. Of course this is no excuse to forget to use your legs to ensure straightness!
(This, and/or the presence of mounted score-collectors zooming away from the fence, or just general muppet-standard riding, cost me a Novice win once!)