Praying in tongues is not something that necessarily comes easily. We may have received the gift, but developing it sometimes seems hard. This is why so many Spirit filled Christians remain in "GaGa Goo Goo" stage. So how do you get to the place where tongues comes as naturally as normal speech? One word. Practice. You just gotta do it.
A 4 Point Plan
1. Determine in your heart that you are going to develop in this area of spiritual life.
2. Make a time each day to practise, and keep to it. Mornings are often good for this, but you find what suits you. Start with 10 - 15 minutes and increase it as your spiritual muscles develop.
3. At the beginning of your prayer time ask the Holy Spirit to help you pray. He is the One that will give you utterance (fill your heart and mouth with the words).
4. Discipline yourself, and pray in your unknown tongue for 10 - 15 minutes (more as your prayer time increases, an hour is ideal). Keep it strong. Make sure you can hear yourself, and don't taper off after 10 minutes. Listen to what you are saying, even though you don't necessarily understand it.
If you will do this consistently for several days, you will gradually become more and more skilled. Over the weeks your spirit will begin to sense the different moods and flows of the Holy Spirit. The result will be skill in the area of tongues. Skill is not an end in itself obviously, but a means to greater intimacy and accuracy in your relationship and spiritual walk.
Don't be discouraged if it takes time. When we are first learning a new language we stutter, mispronounce and often times just sound plain funny.
This is about you and God. Who cares what you sound like? Certainly not God. He just loves to hear you lift your voice to Him in heavenly tongues (even when they sound like a dribbling baby saying "Ga Ga"). Just look at natural parents with their little babies. Their child comes out with some obscure sound that, at a stretch, sounds vaguely like "mama" or "dada", and they are over the moon.
Suddenly, their kid can talk! In their eyes the child is one of the greatest orators that the world has ever known. How much more do you think does God our Heavenly Father enjoy His children when they come to Him. It is so liberating when we realize that not only do we love and enjoy God's presence, He loves ours! Stick with it - pretty soon you'll be flowing fluently in more languages than you can number.