Well my coach and I had a discussion that I should mix up my second serves more by throwing in a slice serve about 50% of the time however I disagree with him because I find that the slice serve doesn't clear the net like the kick serve can, so I just leave slice serves for a first serve mix up. Since my first serve goes in about 50-70% of the time and gives me a lot of easy and free points, I typically put a little extra pace to my kick serves when I am hitting a second serve, especially when I am up during my service game. While I do double fault at least a few times during a match, the effect is quite minimal because I don't really lose points on my service games so I could lose free points here and there. But when I'm playing another top junior, I can't afford to play like that because I need all the points I can get so what ends up happening is my second serve gets picked on because the kick serve will just sit up there or I will double fault trying to go for more off the second serve.
So what my coach has proposed is for me to use a slice serve on my second serve half the time. Obviously I am improving my kick serve everyday, so it isn't like I'm stuck. But to "procrastinate" the situation a little longer, should I be using a slice serve as a second serve?
Sounds to me like your coach has made a great call. As StevoTG says, stick to your winning game with the kick serve when you're having success with it, but if you feel these high level plays are going after it then definitely throw in a few slice serves to disrupt their timing. I feel it works very well for me personally, and it's a great shot to have in your armory, so by all means practice with it and see how you go.
I would recommend using slice on both your first and second serves quite a lot if you're playing on grass or carpet...it won't come back often. Hard courts / clay though, you'll still be wanting to use your kick serve for the second serve 70-80% of the time, the slice just won't be as affective.
Hope this helps. I'm no coach so I could be talking a load of bullshit there, but I'm just speaking from personal experience of what works for me.