Tension Build Up - when to roll
A thing I see happen a lot is people roll dice first, then describe after. It's more fun to freely narrate things, build up the tension, only roll the dice when it would be the worst possible time to fail, AND then see what happens.
"Ok, you've made it to the enemy commander's tent. It takes 10 long, agonizing minutes of waiting hidden under a cart until the squad that decided to start gambling near by decides to go down to the river. As you flip open the tent's flap... roll the dice."
Even though the success isn't changed, it's the fact that by the time you roll the dice, the stakes are significantly higher. If you get caught sneaking in at the edge of camp, you can run away. If you're caught in the middle of camp, you're in it deep.
Tell me how you do it
Let the players narrate how they succeed. Sometimes it's the little flourishes and description which lets you see how awesome the character is, or gives you some important idea about who they are.
"I got an 24 on Intimidate!"
"Well, that's definitely a success, tell me how you do it."
"Since I had already pulled out the knife and got in his face, that happens. But then I show an expression of realization! My eyes widen, I back up and smile. I look over my shoulder and say, 'Oh, yeah. That's right. You were supposed to protect the Don's belongings. And that SURE IS a rare painting he has. He'd be REAL mad if something happened to it, wouldn't he?'"
This can be information, a tool, etc. that the PC gets after succeeding.
"You've disarmed the poison gas trap. You've now got a sealed clay jar full of poison which, if opened, will spray forth a noxious mist. Be sure not to break it..."
"Well! Now that you've figured out the locking mechanism to the gate, you realize all the other doors probably use the same system. It'll be 5 to 10 minutes each one, but you can definitely get them open with some patience."
Don't do this often, but sometimes success brings it's own complications.
"You've done it, you've killed the Grand Assassin, right in front of the meeting of the Thieves Guilds of the West City. Everyone is silent and nervous for a minute, before one of them stands up and shouts, '200 gold a month. That is what we'll pay for your services.' Another jumps up, '250!' and before you know it, it's a bidding war. They've assumed you are, in fact, a corrupt paladin seeking to be the new Grand Assassin."