I am not exactly sure what you want - if you want to change the monsters stats, change the monsters, or what. So I'm going to throw suggestions at a wall and see what sticks.
Perceptions of Pain. This is about making it seem more dangerous/painful than it is.
Focus fire. Having all the baddies ganging up on one guy, especially when he's bloodied. Even making them take OAs just to get to the person taking the punishment. This will make all the players stand up at attention, and try to rescue the guy in danger.
Save your lurkers. They usually have powers of staying hidden, so let them stay until there's a bloodied target worth stabbing in the back. Or, have them appear right behind a caster.
Have your elites hold their action points. Wait until they hit someone and bloody them - then pop the AP, so they pour more hurt on the wounds.
Fiends of a Feather. If you want to change the monsters/encounters, put pair your monsters with abilities that compliment one another.
Ghouls/Chillborn Zombies/Ettercaps all do extra damage to immobilized targets. So allies that can immobilize are their friends (especially something that can immobilize at ranged, or multiple targets). The Dazed, Blind, Prone, and Restrained conditions grant Combat Advantage, which is great for monsters with abilities or extra damage die that key off CA.
Monsters paired with hazards or terrain that is in their favor; dangerous floors with flying monsters, poison or necrotic area affects with undead, etc. Don't do this too often; your players will get very frustrated when everything is working against them all the time.
Do not forget auras. Auras that damage PCs when they enter or begin their turn in them are mean - especially when you have an aura on a melee oriented monster (Chillborn, again).
Features of the Arena. Look at page 67-68 in your DMG. I never hear about anyone using these, but I find them really interesting. Particularly the Mirror Crystal; I want to see that stuff used to its maximum potential.
Monster Garage. If you want to alter their stats, do so. As someone has suggested, look at the typical damage array for that monster, and increase. Monster stats (AC/etc) are also all guidelines; I turned Chillborn zombies into brutes (thus changing their defenses and HP), but left their attack bonus the same as a soldier. (The combining auras were a pain for my party).
Additionally, shop around, look through the MM, and look for powers that you think are more interesting. Or play with a few powers that look thematic for that monster. For instance, take the Blazing Skeleton. It can only do two things - shoot a fireball and claw someone. Boring. I added a death effect - when it died, all that fire and ash mixes with the powdered bone, creating a cloud of ash and heat that lasts the entire encounter in a close burst 2; heavy obscuring terrain, and those who begin their turn take 5 damage in the cloud. Suddenly, the battlefield is changed with this big, hot cloud of ash; monsters might scurry behind it to get out of LoS of mages, things resistant to fire might dive in and try to hide, etc.
Final suggestion: A very simple thing you can do is let a monster do more damage when it's bloodied. Either a static number, or an extra die of damage. If it's a controller or artillery, allow its areas to get bigger.
I've been running a lot of combats, and I've noticed that generally, PCs are very, very hardy. You can throw a hell of a lot at them, and they can still come out. So making the encounters typically more difficult, with the same amount of XP, doesn't change things a whole lot.
