Praying in the Spirit can take two forms. One is the private prayer language that some people take from 1 Corinthians 14:2. In fact, much of this chapter is about speaking in tongues. However, many people who encourage speaking in tongues don't consider everything Paul says about it:

--speaking in tongues should be for the benefit of the church (14:12)
--speaking in tongues should be matched with an interpretation (14:13)
--you need to pray with your spirit and also your mind (14:15)
--it's better to pray five words with your mind than 10,000 words in a tongue (14:19)