This one is just killin’ me. I can’t seem to do it consistently at all. I’ve been practicing on about a 4" (10cm) curb, going up, not down. Seems that if I try to hit it fast, the wheel just stops and I keep going. Major UPD. But, once in a great while, I manage to keep my balance and the wheel will roll over the curb. If I hit it slow, it’s very dependant on pedal position. If my pedal (either left or right) is at about the 2 o’clock position I can power over the curb, sometimes. Not very consistant on this one either. And when you’re just out riding around you can’t count on the pedals being in just the right position. I’ve had the best luck setting the unicycle up with the pedal in the right position, against the curb, then I’ll walk the unicycle back about 8 or 10 yards, then mount and ride up to the curb guaranteeing that the pedal will be in the right position. This just isn’t practical in the ‘real world’ though. What’s the best technique for doing curbs?
Hopping? 
Can do it that way. It’s all about the challenge.
Once you get better at guaging distance, you can accually hop to the spot that will put your cranks in the right position when you roll up to the obstacle.
Honestly, in the “real world” you’ll find yourself hopping over curb+ size obstacles. It’s rough and hard on your rim, to just go slamming into suff.
Agreed. You don’t have to hop, but you have to go through the motions thereof. It’s going to be hard to raise your body that 4" if all you do is just roll into the curb… so just before you get to the curb, start moving your body upwards… as if you were going to hop. Pull up on the handle a bit. You don’t have to get the tire off the ground, you just need you help your uni go up and over the curb. “Up” is the key word here… you’re not rolling over a bump, you’re increasing your altitude!
What maestro described is often called “unweighting” the unicycle. This allows you to ride over stuff that would be impossible otherwise. It’s kind of cheating, but hey, it would be impossible otherwise so it’s okay.
Also you could use a bigger wheel/fatter tire. My Wilder with 24" Gazz will roll up a 4" curb in any pedal position with little problem. I assume you’re on a 20" or Trials uni.
Hehe couldn’t resist 
Lean back just as you hit, the unicycle is going to slow down going over the curb, and that tends to throw you forward. As you ride more, you’ll also get better at making adjustments for riding over the curb. Aim for the curb, but by turning away from the curb and back towards it, you can guarantee you hit it right.
I am glad Underdog brought this up. Rolling up curbs fascinates me, but I don’t do it.
Funny thing, my memory from back in my teens when I learned to ride a unicycle, I remember riding down curbs, and have vague memory of ‘thinking’ how cool and surprizing easy it was to ride up the curb. But I do not have a memory of actually riding up a curb.
Now about 40 years later, rolling up a curb looks impossible. I want to power-roll up them but it confounds me that I can not be sure that the cranks will be in the proper positions to power up. Even on my Coker I don’t roll-up curbs more then 2 inches high.
So my goal this summer is to work on going up curbs. I’d like to roll up curbs on my 24" Muni and my Coker. Since I can hop up a 4" curb, that “unwieghting” up a curb should be a resonable next step.
And then when you find hopping or rolling up curbs are both too easy, you can always try throwing in a 360, a rolling unispin or a crankflip up the curb!!