Style of play
The opponents king is the most vulnerable piece to attack. You could be down on material, and have your position blown apart, but if you mate your opponent's king you will have won the game. The opponent may have completely outplayed you previously in the game, but your mating attack will give you the point.
Simplifications of material are unlikely to put the opponent under much pressure, and the game is more likely to end up as a draw. However if you are constantly putting your opponent under pressure, then they are more likely to eventually break. However apply this idea recursively!. Do not punish their mistakes by direct tactics which could simplify too quickly. Instead use the opportunity to put them under even more pressure. Only when they have made a really significant error should one try and capitalise directly on the advantage and risk losing the pressure exerted on them for concrete gains.
Try to choose an opening which give a certain flavour to the game which suits your style, eg a lot of manoeuvering. Eg the English does not give a massive advantage, but gives a chance to be creative in terms of manoevers. Some openings like the Kings Indian give rise to positions where there opportunities to use pawn breakthroughs. Whereas the Gruenfeld is about piece pressure on the centre, with thrusts later designed to blow up the white centre such as c5.
Move ordering is a technique to consider if you know what your opponent likes and simply avoiding it through the flexibility offered by particular move orders. If you know your opponent plays the Budapest gambit for example, you could start with 1. Nf3. Or if you are black against 1.d4 you could play 1..e6 to leave the possibilities of the Dutch defence or the French defence.
Choice of opening
"Chess for Tigers" by Simon Web suggested this as a method for improving one's results. Again the idea is not to have a perfect opening repertoire, but to have an opening repertoire which brings good results in practice. Openings which suit your style are likely to give you the best results in practice. Play these opening lines to maximise your chances of winning. Try and understand why statistically you are winning in certain openings and not in others.
Don't just copy the Grandmasters if you don't understand what they are doing. Play to your own level in order to have a grip to the motivations behind your moves. If the opponent plays an unexpected move for example, then you are in trouble.
Do not understand just reams of moves. For example in the Ruy Lopez why does the White bishop spend four moves to get to c2? A good reason is that white has the advantage in space and does not want to simplify the position. A bad reason for playing this is that is because one has seen Grandmaster's doing it.
There are several factors usually in the context of a tournament game. For example:-
a) The clock
You cannot leave yourself too short of time to make a lot of moves. In this respect it is sometimes better to play practical moves quickly, rather than try to find the "perfect" move. In finding the "perfect" move, one's clock situation may be worsened considerably.
b) The opponent
You may find out information about the opponent such as they are a sicillian dragon specialist. Do not go into their pet variations. Instead be prepared to play a boring line if necessary to get them out of their book knowledge and make them fight on their own resources.
If you feel tired, do not go in for the most demanding variations. Try and play a solid game, which does not require too much mental effort. It also hopefully means that you wont be blown away after one mistake. Then try and get as much coffee in during the game, to try and wake yourself up.