There are two classes that desparately need exacting knowledge of how a map is laid out, and they're Sniper and Engineer.
However, their goals are different. In the case of an Engineer, you need to know how best to fill an area with aimbot controlled sentry gun bullets, but need to know all the lines-of-sight to your gun, so there isn't an area where a scout can peak around a corner and take your sentry down.
For Snipers, it's just the opposite. You need to know all the long lines-of-site on a level, so you can reach out across the map to shoot things, and you need to know where people are most likely to be walking. An excellent line-of-sight that points to an area nobody walks through is useless; a less optimal line of sight that the whole other team seems to be in the habit of using is great.
You also need to intimately know your sniper nest. What are ALL the angles of approach to you? Can anyone attack from above? Is there a great line-of-sight for rockets to pick you off? Can spies easily approach? When you are out of scope, you can't aim well at distance and you ain't charging, but when you're in-scope, you cant look around for danger. So, you want to optomize your out-scope time; only look at places important, but also don't waste time looking in areas that people can't get you.
MOVE. You should know four or five sniper nests on a level. You start headshotting people and they get peeved. They make it their life goal to have you screaming on the end of a crit rocket. And killcams give away your location. Remember that. And move after a successful run.
Know where the medkits are. Even more important than with most classes, you really need to be a cool frood who really knows where his medkits are. Any shot that doesn't kill you sets you up for a shot that will. If you know how to get to a medkit easily, first off you've got full health again, second that counts as your "move."
Finally, know how to flee. When you're spotted, and you will be spotted, already have a plan on escape so you don't waste precious time deciding what to do. And if you can pull it off, escaping past your local union Engineer's sentry gun will not only get the heat off you, but make your Engie friend really happy.
One final thing that may help (some may consider it cheating...) is to get a mouse with a DPI switch. My mouse can be set to 400, 800, or 2000 DPI. If you set your mouse sensitivity while set to 400, and then switch to 800, you double your mouse sensitivity (halve it, if the other way round). A neat way to give you a boost of sensitivity when you need it
