Notes on being a GM
A high attribute or skill is not an advantage unless a character gets an opportunity to use it.
A low attribute or skill is not a disadvantage unless a character is placed in a situation where it would be useful.
This is an important and underappreciated aspect of stats-based roleplaying. Players should be put into situations that test their advantages and expose their disadvantages. In any given situation consider how the characters can use their abilities to come out on top.
Consider how members of the group can use their abilites to assist other members or provide a breakthrough for the whole team. For example, when the combat master's gun jams in the heated firefight, the tech wiz might be able to fix it. When the team is stuck in a dead end alley with enemies in pursuit, the rogue can pick the lock on a nearby door to provide the team with an excape route.
On the other hand, there are time when characters should be put into situations which expose their weakesses. However, do not let them get stuck, lest the adventure get bogged down. It is better to provide the character with a complication or a missed opportunity.
A character without any mechanical skill who is driving to a meeting may suffer a breakdown, which they are unable to fix. They may need to find alternative transport quickly. Alternatively if they miss the meeting they might lose a job, useful information, or a new contact. If they stood up an underworld kingpin, they may have a problem....
There are two extremes when handling character sheets and other info:
An effective compromise is to let the character keep track of all their character data, but for the GM to keep a partial copy - enough to monitor damage and make a few skill rolls in secret.
At the end of each session the GM should get a complete copy of the PC's stats for safekeeping; for those still using pen and paper excusively, this provides an excellent opportunity to rewrite a new sheet without all the corrections and crossing out.
Describe the game world, not the die result. "You see Bob running away from 3 guards; they are about 20 meters away from him" vs. "You got a 7, Bob is running away from 3 guards, 18 meters behind him". It's usually better to use the PC's point of view rather than describing what "is" happenning (consider "You see" in the example).
Feel free to introduce irrelevant information, especially on poor rolls.
Don't be afraid to be too obvious.
"It's made of metal."
"What type of metal?"
"A shiny metal.".
Never descibe something (or someone) important in more detail than other things unless the characters know it's more important, or it/he/she is really standing out from the crowd.
For example, the PCs are negotiating with a weapons dealer. One of the PCs (with his cybernetically enhanced x-ray vision) suddenly spots that the dealer is an undercover cop. They can't tell the other characters without tipping him off as well.
This is always awkward. It helps if you are playing with experienced roleplayers.
In many cases it's easier (and better) not to bother, and just tell the whole team. For example, you may wish to make perception tests for the whole group (adding a group bonus to the roll) and simply tell the group. Where time and noise is not critical it is likely one character will tell the others.
But in cases like the cheesy example above, the drama hinges upon one character knowing something the others dont. The only really effective way to handle this is to pass them a handwritten note, although the other players will still know that something is up. If you will be doing this a lot, pass around a lot of blank or misleading ones too.
For example, hostage situations, complex plans, etc.
This is always a real pain in the arse. In all cases, if the different parts of the group are not in contact with each other, enforce that on the characters. Don't let them act on knowledge the characters don't have. That said, if it's a modern to high tech setting, its probably easiest to let them communicate with mobile phones or earpiece radios or whatever (assuming the bad guys don't jam their transmissions...).