I'm not a good server and still practicing serves a lot. I can use flat, topspin, twist(weak), and slice serves. Because of the margin of error I get from topspin serve, I'm trying to put some topspin to the flat serve and use for the first serve. However, I just can't do that. I just have to serve either flat or topspin. So I want some tips (toss, motion) about how to serve with pace with adquate amount of topspin.