Isaiah53,
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Well, yes and no.
"Yes", in the sense that spontaneous, or improvised prayers (as far as what one is actually saying) are obviously allowed.
"No" though, in the sense that how one prays, should be informed by God Himself. This occurs through one's teachers within the Church of Christ (the Orthodox Church), and as we progress on the way towards becoming better attuned to the inspirations of God Himself, this is a process which will be informed perhaps quite directly by the promptings of the Holy Spirit - either way, it is the Holy Spirit Who is teaching men how they "ought" to pray, and what they ought to pray for.
For example, one area which shouldn't be left completly to personal taste (at least from what I can gather from everything I've read or been taught) is the "mindset" with which one approaches God in prayer. While there is certainly a paternal intimacy which God has in regard to His children, it shouldn't be forgotten that this same God is also a King (King of Kings in fact), Lord of all, and is the font of all Holiness. If one has such things in mind, then this will naturally translate into how one prays.
Because of the above considerations (and because our faith is not simply an individual matter, but is something shared in common with the Church, which has in fact been the means by which God has given us spiritual life in the first place, and taught us the Good News), the Church has a great deal to say to us as far as guidelines for prayer go, even if we are simply offering a brief, informal prayer, or simply stopping at the prayer corner in our home to be refreshed (though prayers of course can be said anytime, anywhere!).
Thus, there are certain postures of prayer which the Church teaches (generally, we stand when we pray), and even certain prayers which many prayer rules will recommend we ought to pray often. For example, it is good to always ask God's blessing, particularly in the way the Apostles and all of their spiritual descendents have taught/practiced - to pray and make the "sign of the Cross" upon ourselves, for this sign itself is a prayer, and puts fright into demons who know the Holy Cross is their ruin.
So, obviously impromptu or otherwise informal prayers are valuable, but they should also be informed by an ecclessial consciousness (meaning, that we are praying as part of, or as one desiring to be part of, the Church of Christ.)
Obviously, (subjectively speakin) whether one takes the above advice, in large part will depend on what they think of Orthodox Christianity. Objectively, however, that is the state of things.
Seraphim