Classical guitarists play in a very correct fashion; they hold the guitar such that the fretboard is orientated in a straight up/down position, thumb behind the neck, wrist bent maybe around 120 degrees. If they need to see their fingers (which they might when they are training) they move their heads rather than tilt their guitar.
The guitar is often held (the following is for right handed players)with the curve in the bottom of the guitar placed upon the left leg and the foot elevated on a foot stool.
The right arm generally rests on the top of the guitar with the wrist generally dropped over the sound hole. Of course this is for fingerstyle playing but everything I have writen regarding the left hand is equally applicable to other styles.
If you play standing up it could be that you are wearing the strap too long. It is important to ensure that the wrist is not too bent and that the fretboard is in an up-down orientation.
Try to play with the tips of your fingers (unless you're playing a bar chord).
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