Today we're going back to basics with one of the cornerstones of being a Dungeon Master- making stuff up on the fly. As stated before, it's important to make sure every character is active in the story, a big part of which comes from characters getting to use their specific skills, equipment, spells, and so forth. So, it's important to have an idea of what each character can do (that goes for players as well- DON'T FORGET what your character has on them, you never know when it'll be helpful). Some DM's will even keep copies of everyone's character sheet. If you need that reference to be prepared, go for it, just make sure to update it when characters level up or get any significant items.
Now, an obvious issue with this is that you don't want the situations where characters get to strut their stuff to be too contrived, it's important to weave them into the story as naturally as possible. Perhaps every once in awhile, set up scenarios that are geared toward a specific character, such as when the party is heading toward a particular character's goal. But, even in these scenarios, keep the other players in mind as well, for one very specific reason. Most games are based on, at least to some degree, random chance. So, just because the party's monk has 15 points in balance, doesn't necessarily mean they'll succeed in crossing narrow passages every time. Thus, it's best to think outside the box. Construct situations where more than one character has the capacity to resolve the conflict. If the party needs to get into a building, it might make the most sense to construct the scene around the rogue sneaking around and picking the lock, but make sure there's alternative options in case something goes wrong or the party doesn't see your solution right away. Have someone around to talk to so the bard might be able to talk their way in. If someone has a grappling hook on them, perhaps note how high the building is to subtly suggest that they could potentially climb to the roof. Characters with spells have a huge amount of problem-solving tools, so if everyone's stumped, perhaps say something like "anyone have magic they could use?". With time and thought, a wizard can come up with some zany way to finagle their spells into something useful.
And speaking of which, if the party comes up with a different solution, be ready to roll with it. Remember- cooperative storytelling is the key. If the party comes up with some goofy way to get into the building, let them as long as it makes sense. Heck, even if it doesn't, let it work anyway if you think the game could use a good laugh. And, if all their attempts truly fail then you might need to just hand-wave the situation with a little "hand of God". Maybe a guard hears the commotion caused by the party's multiple failed attempts to get inside, opens the door to see what's up, and BAM we've got a fight. The world shouldn't be doomed to be under the evil king's tyrannical rule for all time just because the party's bad luck kept them from opening a door.