Each PC (and player) is motivated by something. As a GM, it should be your obsession to uncover that nugget of desire and use it as a carrot for your players.
For example, in our RotRL campaign, Niko has a seemingly unquenchable thirst for knowledge. Sprinkling in some lore, extra room tidbits, or mysterious secrets help to satiate that thirst in small doses. The player behind Niko shares the same love of puzzles and information. This overlap makes the GM’s job easier.
Silas, on the other hand, hasn’t revealed much of his backstory. The main slice of flavor that we’ve gotten is his compulsion to be top dog on campus. A former champion of his tribe, Silas will not allow anyone (man or beast) to best him in combat or be superior to him in any way. Exploit this in combat. Have intelligent creatures challenge him outright. An NPC can engage him in a duel. Fuel the fires.
There aren’t many details for Krask either, apart from the recurring theme of <b>GOLD</B>. Motivating PCs with loot and treasure is a tale older than time; shiny swords and piles of coins will always entice players with their siren song. Krask dragged a masterwork horsechopper around the battlefield until he had an opportunity to sell it. And thus, future adventures could be funded.
Each player will be drawn to different aspects of the game. Curating the adventures to include a personal touch will help prevent the campaign from becoming stale. It will also nudge your players into coming back every week; they crave the carrot at the end of the stick.
Image from Stuart MacCabe : http://stuillustrates.tumblr.com/