Technical analysis assumes that:

All market fundamentals are depicted in the actual market data. So the actual market fundamentals and various factors, such as the differing opinions, hopes, fears, and moods of market participants, need not be studied.
History repeats itself and therefore markets move in fairly predictable, or at least quantifiable, patterns. These patterns, generated by price movement, are called signals. The goal in technical analysis is to uncover the signals given off in a current market by examining past market signals.
Prices move in trends. Technicians typically do not believe that price fluctuations are random and unpredictable. Prices can move in one of three directions, up, down or sideways. Once a trend in any of these directions is established, it usually will continue for some period. The building blocks of any technical analysis system include price charts, volume charts, and a host of other mathematical representations of market patterns and behaviors. Most often called studies, these mathematical manipulations of various types of market data are used to determine the strength and sustainability of a particular trend. So, rather than simply relying on price charts to forecast future market values, technicians will also use a variety of other technical tools before entering a trade.
As in all other aspects of trading, be very disciplined when using technical analysis. Too often, a trader will fail to sell or buy into a market even after it has reached a price that his or her technical studies identified as an entry or exit point. This is because it is hard to screen out the fundamental realities that led to the price movement in the first place.
As an example, let's assume you are long USD vs. euro and have established your stop/loss 30 pips away from your entry point. However, if some unforeseen factor is responsible for pushing the USD through your stop/loss level you might be inclined to hold this position just a bit longer in the hopes that it turns back into a winner. It is very hard to make the decision to cut your losses and even harder to resist the temptation to book profits too early on a winning trade. This is called leaving money on the table. A common mistake is to ride a loser too long in the hopes it comes back and to cut a winner way too early. If you use technical analysis to establish entry and exit levels, be very disciplined in following through on your original trading plan.