To keep the ball low, it helps to get low. If you are contacting the ball at a point that is higher than you want it to go, it will bounce higher than you want. Placing something over the net so that you only have a small window between the net and what is above it and practicing getting the serve between the net and the obstacle forces you to keep the serve low. This is a video that shows what I mean.
There are other videos that show people who have set something up so you do not need someone holding the racket over the net. But notice the hight of the contact point for Liu, Ma and Wang. They all contact the ball pretty low.
If you are serving from the backhand side of the table and using pendulum type serves, every so often giving a no-spin or topspin serve short to the deep forehand gives you and easy setup as long as the no-spin or topspin look like underspin. Mixing things up so you are serving long with wide angles to both sides, and at the crossing point, and short to the crossing point, and to the deep forehand and backhand but not giving easy serves where the ball is not long or short, will keep your opponent guessing where the ball will go next. But it is worth having an idea where your opponent will put your serve back so you are thinking two moves ahead and can respond to what your opponent does more effectively.