Plan to Improvise
You don't want to have everything planned out in detail, but you do want to have enough planned so you are ready for most likely actions the Heroes will take, and have enough interesting twists to make the encounter fun. You want to plan out the goals and tactics of the NPCs and most importantly, provide lots of stunt-able features in the environment.
Here is a planning template I use:
Connection to PCs
Of course, the scene must be connected to the Heroes somehow. Normally this is fairly obvious, but sometimes, you might want to make it clear. Ask yourself, "What is the purpose of this scene in the adventure?" or "Why are the Heroes going to participate in this scene?" Ideally, the scene should be about something more than just killing the opposition and taking their stuff.
Once I understand the reason for the scene, I try to pick an exotic or iconic location. The setting of Heirs to the Lost World Is rich with great locations for set-pieces. Just on the swashbuckler side of things gives you:
Ship boarding action
Classic tavern brawl
Dungeon or Inquisition torture chamber
Horse and/or carriage chase
Completely of partially wrecked ship
Of course, for extra coolness, mix in a little Voodoo, Aztec, Maya, or American Indian elements.
Next, plan out your major NPCs in the scene. For each, be sure to write down a motivation and a short, easily distinguishable hook to aid your narration. In this step, do not put the NPC's goals in the scene, but rather their overarching motivations. This will help you decide their actions when things don't work out as planned. I normally start with the following categories for my major NPCs:
other (perhaps a repeat of one of the above NPC types or something different altogether)
Of course, do not feel confined by these categories or your set pieces will start to look too similar.
Before starting the set piece, make sure the goals of everyone are clear. This includes the PCs and the NPCs. If unsure, consult the character's motivations when developing the set piece.
NPC Actions and Tactics
Using the NPCs motivations as a guide, think about their goals in the scene. How will they attempt to accomplish those goals? You may want to think about a couple "if, then" statements to react to likely actions from the PCs. Also, be sure to think about when the NPCs will surrender.
Stunt-able Features: "Action Scenery"
If you want the players to use the environment in clever ways, you must pre-plan potential stunt-worthy items and call them out when describing the scene. If using a battlemap, draw these items on the map or use tokens to represent them. Classic items include, ropes, chandeliers, wagons, awnings, etc. but anything will do. Try to have potential ways to jump, climb, swing, slide, throw, drop, etc. For example, a for a scene taking place in an Inquisition torture chamber, be sure to describe all the torture implements and mark them on the map. You know they will be used in some great stunts. Another technique you can use is to place items with unspecified contents, allowing the players to spend Destiny points to decide. For example, you could draw some crates and barrels on the map while hinting or reminding how the Destiny point rules work.
When thinking of the terrain for the set piece, variety is the key. Try to avoid the "10 foot room". Multiple levels are always great (balconies, stairs, etc). Try to have terrain that allows for the use of non-combat skills like acrobatics, athletics, and swim. Also, add something to change things up by including an environment complication. This could be a change in the weather or some other natural change that occurs in the middle of the scene. Alternately it could be a hazard somewhere nearby or other feature that provides tactical options. This could be a choke point, something that slows or speeds up movement, a potential danger like a fire or pit, etc.
Most set pieces need additional NPCs. A good rule of thumb is to have at least one intriguing henchman and a couple types of mooks. To help drive the action, it is best if at least one NPC has a ranged attack. As before, do not feel limite by these categories.
Nothing builds tension like a burning fuse. Not all set pieces need them, but a time-sensitive complication can be great. It does not need to be a scene-ender (like a fuse leading to a bomb that kills everyone). That would not be much fun. Rather, have a "clicking time-bomb" that will release a complication on the scene if the Heroes do not deal with it. For example, they see a rope that is starting to fray and will break soon, dropping _____, or they notice a small fire starting to spread. Alternately, you can just have a planned twist mid-scene. This could be an environmental change mentioned above, or something completely different. For example, maybe the ship they are on starts to sink or maybe the militia arrive.
While perhaps not appropriate in every set piece, moral complications can make the scene more than a simple battle. The two classic examples are the innocent bystander and the possessed friend acting as enemy. To think up moral complications, try to tie them to the PCs' personal motivations or complications. Sometimes, they may be played more for the humor value (ex: an alcoholic PC witnessing a rum barrel slowly leak) or can be much more serious (ex: chained up slaves trapped on a sinking ship). While it can be difficult to plan, it is also fun to come up with conflicting moral complications between individual PCs or for an individual PC, forcing difficult choices.
Think of potential ways the scene could end and come up with a short plan in each case. I have found that most scenes only have a handful of potential outcomes. By planning for them all, I prevent myself from subconsciously railroading my players to a particular one. You don't need huge amounts of planning, just jot down a few ideas for each possibility. In addition, plan out any rewards the PCs will get at the end of the scene. This could be treasure, information, contacts, etc. As always, be sure the adventure can still proceed somehow regardless of the outcome.
http://www.obsidianserpent.com/downloads.html - Downloads page on ObsidianSerpent.com, check out this Set Piece template.
http://www.rpg.net/news+reviews/columns/action07jul03.html - The first of a series on action scenes by Dan Bayn
http://arsludi.lamemage.com/index.php/48/anatomy-of-an-action-scene/ - Anatomy of an Action Scene
https://sites.google.com/site/amagigames/action-scenery - Action Scenery