Here are some ideas I've pondered for long encounters:
Terrain encounter powers: These can keep things fresh. If someone knows that there is a terrain piece they can use with great accuracy for more damage than an at-will, plus some effect, they will use it. A moving fight can travel through different terrain features that could be used.
For instance in a warehouse, an enemy might try to hide behind a heavy crate, and you could push the crate pinning him with a restrained save ends, and doing some damage. A crate hanging from a pulley system could be dropped on top of an enemy, dealing damage and knocking them prone. A barrel could be rolled down a narrow isle, and enemies might be forced out of its way or take damage. A barrel of explosive could be ignited.
During a fight in a dwarven hold, there might be blood stones that allow you to do extra damage if you stand on them while bloodied, there might be a horn of the dwarven kings that when blown gives some leader type bonus and a few free basic attacks. There might be a banner which can be waved to activate its magic and heal some allies, while giving someone a basic attack.
The key is to make all such encounter powers more attractive than an at-will the PC's have, otherwise, they will not be used.
NPC encounter powers: NPC allies can increase the potentially interesting things a party can do. For instance the party may be trying to rescue an NPC baroness. The baroness may have magic at her disposal to help out once she is rescued mid-fight. She may grant healing, she may chant a tune that allows extra damage, conjure a zone that can be used by the PC's to push enemies into for some effects or damage. She may invoke memories allowing a PC to recover an encounter attack power. She may allow someone to take an extra action with her action point.
Break in the fight: Sometimes there is a break in the fight, as the enemy takes flight. PC's may give chase, and you could be looking at a skill challenge to keep up with endurance, force your way through brush with athletics or find an easier path with nature and perception, a heal or nature check to recognize a spring of renewal that refreshes endurance DC's and lets everyone spend a healing surge and refreshes an encounter attack power. At the end of the skill challenge, you could declare a milestone as the PC's catch up with the fleeing enemies and join battle again, though the enemy has met up with reinforcements.
I've been in some 20 round fights. I think I was out of encounter powers and dailies on round 6 or 7, and used at-wills the next 13 rounds. The encounter area was huge and we were running around all over the place, so it wasn't exactly boring. I even used a bull rush once to knock an enemy into a river. There was a lot of climbing, jumping, taking cover, etc. So even a string of at-will or basic attacks don't necessarily have to get boring.
Occasionally such big fights are a lot of fun. But it's best if they are not the norm. A 5-7 round encounter is I think an ideal length for using some encounter powers, some at-will's and maybe a daily. It also keeps the story moving at a more brisk pace.
So within a level or 2 I should be playing a wizard and facing down the hordes of hell for escaping
