The sidebar on page 55 of the SR4 rule book goes into this type of thing.
QUOTE (SR4 pg. 55)
EYEBALLING MODIFIERS
Let's face it: while modifiers help to add realism to a game, they can also bog down gameplay when you have to consult a big list of possibilities and do a bunch of math. If you seek a sleeker and more fluid style of play, try one of the following options:
Adjust the Threshold: Rather than counting modifiers, tell the player to make a standard test without modifiers and simply adjust the threshold to account for how you think modifiers would affect the difficulty. Note that this only works for Success Tests and Extended Tests.
Use the Most Severe Modifier: Instead of accounting for every potential modifier that could affect a test, quickly identify only what the most severe modifying circumstance is, and just apply that one. If it's a situation where you feel a lot of additional modifiers may apply, increase it by 1 or 2 according to your gut feeling. This should allow you to seize upon a single modifïer quickly rather than accounting for all of the possible affecting elements.
For example, let's say you want a character to make a Perception + Intuition Test to notice a clue left in a room. Rather than looking up the Perception Test modifiers, the GM decides that the biggest modifying factor is that the room is dark, and applies a +3 modifier for that alone. If a lot of other modifiers might also apply (the character is wounded, the clue is partially hidden under something else, the character knows what he's looking for, etc.), the gamemaster can simply nudge the modifier up to +4 or +5 depending on his 'eyeballing' of the situation, rather than looking all of the modifiers up.