Two-way train station! Whichever direction you come from, hit the switch, and it'll keep ya boosting on in the right direction. Or, you can place your cart on the slope leading into whichever direction you want to travel, and hit the switch to start going. Or get out at this stop! It's totally a train station!
PS. I'm sure this exact design or one very much like it is all over official forums, but having never worked with rail or redstone before, figuring this out by myself made me happy. And two-way boosters are super easy now.. its just a piece of powered rail.
Now if I could just find a way to simply get power under a rail.
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"If a job's worth doing, no further justification is required." - Alice: Madness Returns
"It takes just as much work to achieve failure as it does to achieve success."
For always on, dig two blocks down, throw down a redstone torch, put a block above then powered rail on top. There are some quirks, like in some places you can't have a self-powered rail up against a block, or you can't have one just before a corner - needs to be a gap between them. (although, switch-based powered rail works in these places).
Example with carved out rock showing the workings.:
For switch based, you just need to put a redstone circuit on top of a block underneath your switch, and then create a path to two-blocks underneath your powered rail (so the end of the circuit is where your torch would be in an always-on situation). The vertical redstone circuits will build themselves, so just build the horizontals.
Example with carved out rock showing the workings.:
Addendum:
For "starting areas", your first piece of track should be switch-based so your cart doesn't take-off as soon as you put it down. Then a gap of one or two unpowered tracks, then some self-powered tracks to get you up to speed. If a powered rail is up against a block (switch-based powered), it'll send you in the direction opposite to the block.
In this screenshot, ignore that the circuits are on top of the ground. This was my first attempt at powered rail, but anyhow - it shows a starting area. You put your cart down on that first inactive powered track, hit the switch, and it sends you into the always-on tracks and boosts you on your way. Intermediate stops along the track should be done similar to the train-station in the first post below, but at each end of the track you'll need a "stop" like this. (The unpowered powered rail also acts as a brake, to prevent damage)
FYI: Here's a video of this track in use, forward and backwards.
Very nice. I believe that placing a powered track next to another powered track can have the same affect as a red torch, ie. if one track is on, it'll turn the other on.
===*===
===o===
So * will turn o on, but I haven't tested this. Just something to consider in all the confusion of making centralised stations.
The next thing Notch needs to add is vertical tracks, so people can do loops.
Powered Rail
And two-way boosters are super easy now.. its just a piece of powered rail. 
