The first step is not to join a poker table that is crowded too much. It could be tactically disadvantageous to negotiate so many players at a time. Also, the lack of elbow room would mean that there is someone near you who is taking occasional peeps on your hands. That is, playing with one's cards close to the chest would be impossible then.
Watch the cards: You can't claim advantage in a poker game by just focusing on your own hands. In order to frame strategies, or to bet, one needs to have an idea about the other player's hands and must be able to read their thoughts, at least vaguely, from their body language. A player who gets two hole cards may not be able to suppress a smile. An intelligent opponent will take cue from that smile and bet or fold accordingly. If you don't have a great hand, observing others will help at least to avoid a disaster.
Learn the art of bluffing: Bluffing has a big part in poker. If you can convincingly bluff to other players, you have a good chance of staying afloat even if your hand is not a convincing one. By convincingly bluffing, it is even possible to get players with decent hands to bow out/fold meekly. Some view it as a negative strategy. But, then, in a hard fought Texas Holdem poker game, sometimes it is required to survive the weather.
Betting: It is the most crucial or the defining element in Texas Holdem poker. There is no fixed strategy for betting, but properly betting can win games in a big way. Having said that, its converse is also applicable; that poor betting can make a mess of one's game. For example, if somebody starts off by betting aggressively from the very beginning itself, that'll tip the hat. Other players, without realizing that he is bluffing, may not take the risk to increase the size of the pot. In certain instances, it is tactically good to call in order to keep others in the game – preventing them from folding – and reduce your own investment on the pot. In short, mastering the art of betting is very vital in Texas Holdem.
No offensive strategy is complete without a plan to back off in place should the losses are huge. The same is the case for Texas Holdem as well. If things are not going well for a player, sometimes, it'll be a good move to fold one's hand in order to lessen the losses. Unfortunately, there are no short cuts or rules of thumb to learn this trick. This gut feeling to fold is instinctive and that comes only with experience in playing Texas Holdem.