Tip #2 - In singles (and doubles to an extent), the pokemon you choose and which one you decide to send first is half the battle. Most people will look at the other person's team and choose a team consisting of pokemon that will counter the opponent's team as a whole the best. That is many players' downfall. The best way to select your team is to try to think of it from your opponent's perspective and try to guess what they will pick to counter your team as a whole. This can be quite difficult when not knowing what items, abilities, or moves the pokemon of your opponent have, but experience will quickly teach you what the norm is, and you can use that information to anticipate.
Note: If your opponent also chooses their pokemon like this and you try to anticipate that, a very repetive inception mindgame will occur and you will wind up becoming very confused. If you anticipate your opponent using a similar method for choosing their team, simply combine the normal method (choosing pokemon that counter the opponent's team as a whole) and the aforementioned method to select your pokemon. The best way to determine which method your opponent will use to choose their pokemon can be somewhat determined by their rating or amount of wins.