There are a couple of ways to avoid a system where players always use their best per encounter powers on the first round of a fight, and work downwards from there until they run out.
The simplest is to create a trigger condition that must be met before a particular power can be used. One power might only work on bloodied opponents. Another might only work if the character has dropped a foe earlier in the round (see Cleave). A third might be usable only if the character using it is bloodied.
Another way is to make powers highly situational, and based upon battlefield location. A parry that redirects an attack from one opponent to another opponent might require both opponents to be in melee reach of each other, and of the character. Such a power wouldn't be usable until that situation was arranged.
Attacks that work well with teammates have to wait until the teammate is in position. For example, an attack that knocks foes backwards might combine well with a teammates area of effect ability. It wouldn't be wise to use it until the teammate was ready and in position, though. So it wouldn't be used immediately.
Effects which work like the Power Word spells aren't usually used first in combat. An enemy has to be damaged first.
Still others could work on a combination system. This can be done stealthily, avoiding a video game combo feel. For example, a fighter might have a special attack that is particularly effective against a flatfooted opponent. This would lead to it being used situationally in the beginning of fights. But if he had another special attack that knocked an opponent off balance rendering the opponent flatfooted until the end of the fighter's next turn, it creates a second use for the first attack that happens later in the combat.
There are probably more ways to do this. But the general rule is still going to be to use your best attacks first, since doing so lets you drop enemies faster. And since the best way to avoid taking damage is to kill enemies as quickly as possible, this will be the predominant strategy.