Well, if you want them to fight things higher and higher above their level, then you need to look at what higher levels give them that helps them do this:

More +hit
This comes primarily from half-level bonus, but also from ability scores and item bonuses. You're supposed to get about +1 to hit per level, to keep up with monsters, so if you want them to keep hitting, you need to give them another +1 to hit for each level you increase the monsters by.

More damage
This comes primarily from better powers, but also from ability scores and item bonuses. I'm not sure exactly how damage is supposed to scale, unfortunately.

More HP
HP comes from levels, pretty much directly. Each level gives 4-6 HP, depending on the class, so just give them that much for each level you increase the encounters by.

Which of these you increase depends on how you make the encounters more difficult, though. If you're just increasing the level, then +hit is going to be the main thing that needs to go up, though damage and HP will be important too, if secondary. If you're just increasing the amount of monsters, and not level, HP will be the most important thing to increase, and you shouldn't give out much +hit, if any, or the players will just hit all the time, while damage should probably go up some, but not too much, so enemies go down faster without becoming effectively minions.