When first learning to play chords:

• make sure all the notes of the chord are ringing out
• Relax your hand
• push your wrist forward, this helps give your hand more space
• use the tips of your fingers
• make sure your fingers are not muting the other notes
• when changing chords, dont take your hand off the fret board, use the shortest route possible to get to the next chord

When playing with distortion:

• mute all the strings that aren't being used
• use both hands to dampen the strings
• use the tip of your first finger and the palm of your picking hand to achieve this

Strumming chords:

• use both downstrokes and upstrokes
• Practice missing out some strokes but always keep your hand moving
• remember to always tap your foot as you play, this will help keep you in time, and will be invaluable if ever in a band situation