Haha that is a difficult question! My players are level 25 and I still struggle with making encounters good
(although it's a lot harder in epic tier)
Here are some tips:
1. Follow the advice in the DMG1 and DMG2. They are really good. Use the XP budgeting rules.
2. Never use monsters more than 3 levels higher than the players. The defenses will be too high, causing the players to miss too much, which isn't fun.
3. USE THE MONSTER MANUAL 3 MATH. This means either use all monsters from MM3 onward (MM3, Monster Vault, etc) or use the guidelines out there to adjust the damage and defenses to the MM3 values.
4. Use a variety of monster "classes". Soldiers are interesting because they "lock down" players like tanks. Artillery are good because you want ranged guys that force your players to have to move around. Lurkers and Skirmishers are good for burst damage. Brutes are neat because they're pretty beefy and hit hard, but don't use too many.
5. Use minions. Use a ton of them. This is a big mistake I made in our early days. I didn't like them because I thought they died too quickly. But that's a GOOD thing because you want your players to feel bad ass! They're the heroes! I recommend using two groups of minions (in heroic that's 10 minions I believe) meaning you'll use 3 standard monsters and 2 groups of minions. 5 standard monsters is just too many hitpoints and takes too long to take down.
6. Make your combats dynamic. A quick and easy way to do this is stagger monsters. Don't reveal them all at once! I like to do this with minions especially, because it's easy to toss on four more minions every round. I used to think of standards as the grunts, elites as the "elite" monsters, and solos as the bosses. Once I realized that minions
are the grunts, Standards are the elite soldiers, and Elites are the bosses. Solos are only for very, very special guys like dragons.
7. Avoid the following: Solos (until you really understand how to make combat fun, they will just plain not be), Elites except for very special circumstances, and action denial effects like dazed and stunned. You can use all of these when you better understand what makes the game fun, but for now it's safe to avoid them all!