The current rules don't allow you to cover up the ball when serving, but there are still legal ways to make it harder for your opponent to see what kind of serve you're giving them.
Serve close to the body.
It's harder to see the serve and your racket motion when the ball is only a few inches from your body (the follow-through will often be covered).
Change your follow-through.
Instead of following through naturally after contacting the ball, change the direction of the follow-through so your opponent can't use your follow-through to determine what type of spin (and strength of spin) you're creating. Almost all of the professionals change the follow-through, especially on their forehand serves.
These are sub-pages of the Hiding your Serve (Legally) page (a child in the hiararchy).