Gah! I was trying to find a post I left on the Beta forums about exactly this topic, but the Beta forums are gone!
Anyway, here are some guidelines I use:
1) You modify the base difficulty if there is something inherent to the task itself that is more difficult, if performed in ideal circumstances or environment.
2) You upgrade the difficulty if there is something inherent to this particular attempt at the task that is making it less likely you will succeed (or more likely to have a catastrophic failure), than the general case.
3) You add setback dice if there is something about the environment or circumstances of your attempt at the task, not inherent to the task itself, which would hinder your chance of success.
1) Task: Shooting at an experienced Bounty Hunter, through a smoky battlefield. The base difficulty is based on the range to the target and silhouette of the target, as the inherent difficulty of hitting something vaguely humansized and shaped at a given distance is constant to the task. You upgrade the difficulty based on the skill level of your opponent, as this is something inherent about this particualr attempt at the task (as opposed to the general task of hitting a humanoid at medium range). Finally, you add setback dice for the environment (smoke), which isn't directly related to your shooting, but still makes it much less likely you will hit your target.
2) Task: Attempting to convince the populace of Coruscant to aid you agianst the Empire without reward. The base difficulty for trying to convince a group of people to aid you would be based on their attitude towards you and what you're offering in return. This is upgraded due to the heavy imperial presence and support on Coruscant, making it more diffcult to convince Coruscant residents to do anything which might attract the Empire's attention and/or wrath. Finally, there is an Imperial agent who has been spreading rumours about how untrustworthy you are - this is an external factor to the task itself, which nontheless has a negative impact on your roll, hence adds setback dice.