These are a list of some of the things i try to do:
Breath deeply and slowly as this should help you relax

• Take alot of practice swing before teeing of (nice and slow) to get you flexable
• Don't put pressure on yourself and keep positive
• Finnaly for the first swing i do on the tee box i never try to swing slower as you can become over relaxed and lose concentration. Instead i try to just swing the club about 80% and follow through at normal tempo

hope my tips help you