World Champion Mikhail Tal once said:
“Later, I began to succeed in decisive games. Perhaps
because I realized a very simple truth: not only
was I worried, but also my opponent”
Remember that it is normal the have some pressure during a game - I remember even games I've played against my computer where I am analysing on a chessboard set up beside it and choosing moves that way leaving me feeling buzzed afterwards.
It will help to have a snack or a drink by you, so that there is something to help calm you down if you need to take a break from the board.
When it comes to making moves make sure you have a system and FOLLOW IT. It should look something like this:
1.Look at opponents move and write it down
2.Check for (new) threats
3.Check for (new) opportunities (for tactics)
4.Consider potential candidate moves (think about strategy if it is a quiet position)
5.Compare variations and choose 'best' move
6.Double check for mistakes in your move
7.Play it, write it down, hit the clock
8.Start thinking about the strategic factors in the position during your opponents move
If you follow a system it will help 'force' you to be productive, even if you are feeling emotionally agitated.
The psychology of chess is not to be underestimated and remember that the condition of your body plays a factor too. Make sure that you are physically fit (brief exercise in the morning before the game can get your endorphins going, but don't wear yourself out!). Make sure you are hydrated and eat a moderate meal 1-2 hours before the game (don't eat a big meal right before, because then your blood flow will go to your gut to digest the food!). Make sure that you are hydrated, keep some drink at hand.
Try to avoid drinking caffeine if you are already feeling agitated. Consider having herbal tea to help calm you down. Do NOT drink alcohol to relax (pretty obvious right?).
As for time trouble, the best way to deal with time trouble is to avoid it in the first place. Make sure you understand the openings that you intend to play. Don't get stuck on non-essential moves. I have seen many times opponents who have only 1 legal move (due to check) sitting there 'gifting' me with the time from thier clock thinking over the position (even in Blitz!). Once it is clear that there is only one possible move you should play it immediately! Then use your opponent's time to think about what comes next.
Also try taking the focus off yourself during the game - try reading about past champions and masters and identify role-models when it comes to staying cool and calm during the game. Good examples may be Capablanca, Botvinnik or Petrossian. Botvinnik even trained himself to ignore his opponent's smoking by practicing games whilst a trainer blew cigar smoke in his face! Try to emulate a role model's attitude to the game, or try to play the 'board' rather than the opponent.