A horse can feel slight shifts in weight, although young horses may need large shifts which become smaller in order to understand them. Simply rolling on your hip bones should be enough. Movements such as putting your shoulders back/forwards, turning your toes in, moving your knee, turning your eyes.. they all change your weight distribution. Mental imaging also subconsciously changes your weight so thinking about walking can often slow a horse down, thinking about cantering can often make a horse canter, and thinking about the horse taking off can often cause just that.
So in order to maintain good equitation, you only need small shifts in weight. They shouldn't upset your balance or your seat. Young horses and/or green horses may need larger aids so they can recognise them, but these should get smaller and more subtle along the way. Older horses that should know better may need a bit of reinforcement.