Refs: the Rewards

What Motivates the Characters in this Scenario?
What's the Return on Investment? Fortune and glory? Fulfillment of a holy quest? The range is huge, but advanced players will bring their own character-related motivations and fulfillment. Encourage it. Work with it. If they're role-playing it: reward it.

Profession.

 * The character is a professional operator (whether spook or soldier), and the government/company/group has some level of interest in the matter, therefore it sends the character.  This may or may not match, or might run parallel to what the scenario was designed for.  I have designed several find-and-grind operations for counterterror teams to have fun with.  Alternately, I played a mercenary in a group that was hired to break in and liberate a few items from a competing Corporation.  The whole group was just coming together and didn't suspect that I was undercover security for the other corp.  Oddly enough, there would be little vignettes that would involve two or so characters, and when the rest of the players came back in the room, there had been a terrible accident to one  of them.  I killed off five of eight before they were on to me, and another two before the last man killed me.  But I had gobs o'fun just seeing the shocked looks on their faces.

Money.

 * It is the easiest route to assume mercenary (free-lance) motivation.

Honor.

 * The characters are faced with a challenge, possibly by a microscenario or ruse to set it all up.  Careful, because this one lends itself to that rut.  A few characters might be motivated simply by sense of adventure (rare), or helping another character-friend, but don't bet on it.

Revenge.

 * Unless a player is willing to tailor himself to let his history reflect some particular event (which I do not recommend), this becomes hard to implement.  If the player is okay with having his history changed, fine.  Possibilities include murder or kidnap of family members, close friends, revered former Commanding Officers, etc.  Otherwise, it can be a great motivation for creating a future scenario after a character does wrong or gets wronged.  Either way, avoid adventure through coercion.

Samaritanship.

 * Doing the deeds because the players feel it is right.  Not a good factor for most, but a very powerful one for some.  This is basically what drives vigilantes after their immediate thirst for revenge is satisfied.

Self-preservation.

 * The characters were at an incorrect place at an inconvenient time.  They witnessed something go down, now the bad guys want to tie up loose ends.  A handy motivation and also a great way to strike back without too much moral and ethical dilemma.

Curiosity.
Those are the basics of character motivation, especially for the first scenario.
 * Find some aspect of a mission that meshes with a high skill of the PC and tell them they have a chance to be around this and get a glimpse of the whatever is a really fascinating subject. It might be valuable knowledge in the long run. If possible, try to make it actually valuable sometime in the future. Afterall, if we can make 'em buy, and it pays off, they get suckered in that much easier later.

Subsequent scenarios usually follow the initial format, but motivations can evolve as the characters and party do. What starts out as professional duty could turn into revenge after a party member is killed...