Agree with the others. I used to try and bomb my first serve and then have a "Cinder-fella" second serve that looked bad and got crushed. I developed a good kick serve I could rely on for a decent second serve and will even use it to mix things up with a first serve when I get tired. I think practicing the heck out of a kicker helped my flat serve too since I was very conscious of my toss and technique.
Head Microgel Radical MP, lead | Babolat Xcel French Open 17ga @ 58