A good rule of thumb is to come up with a general concept, rough it out a bit, then fill in the sheet and the background together. A lot of people seem to run into trouble because they flesh out this complete backstory, then find out that they can't translate it into a set of stats and numbers that matches their background. Ground your character's background in hard numbers, so that what is on the sheet matches what is in the background. Realize that not every character idea that you have is one that would be viable for this particular rule set.
Be aware of what the game itself encourages, and discourages. Shadowrun is a game of magic and high tech, so those two things give characters cheap, powerful boosts in ability. Shadowrunners are teams of hyper-specialists who do incredibly dangerous work, but they are also mavericks. It is unsurprising that it is far easier to make a specialist than a generalist in the game, and points are also tight enough to encourage making flawed characters - dump stats, negative qualities, these are part of the game design.
I agree with James McMurray that above and beyond the game system, also keep the campaign in mind. Don't bring your troubled runaway corporate kid who can do a bit of hacking, into a game of stony-faced cold professionals. Don't bring your stony-faced cold professional, with his move-by-wire and Ares Alpha, into a game of street punks surviving in the Barrens.
Shadowrun is an open build system, which allows an incredible variety of characters. The flaw, though, is that it is very easy to botch it up. The system encourages specialization, but don't make a character who is only good at one thing. You'll be bored the rest of the time, when you don't have anything you can do. On the other hand, don't overgeneralize to the point of uselessness. It can vary from campaign to campaign, but have a dice pool that you have a good chance to succeed with. Taking some lower stats in a few areas, or some negative qualities, can help you flesh out your character, but be careful. You want disadvantages that are fun to roleplay, not ones that are crippling. Remember the point about magic and technology - if you don't take advantage of either, your character will likely be very weak compared to those of other players.