Bed leveling method? Is there a good method or tool to level the bed of 3D printers? I find myself making small adjustments a lot and it's mostly just trial-and-error. A normal bubble level is of limited help and trying to figure out if the head is the thickness of a sheet of paper from the bed in all corners is beyond the capability of my vision.
There are several ways.

1) Simplest is the business card/paper method.

This video shows how simple it is: Leveling the platform

You should feel the same amount of resistance between the hotend and the bed on all sides. On some 3D printers there are 3 screws holding the bed (eg. Solidoodle) and on others there are 4 screws on each edge (e.g. Prusa i3).
On some 3D printers you need to rotate a screw with a screwdriver to adjust it (e.g. Solidoodle 3) and on some you have a nut (mostly wing nut) (e.g. Solidoodle 4).

I find most people move the extruder around when adjusting the bed by controlling the motors. I think the quicker way is stopping the motors (in Repetier Host it's the "Stop Motor" button) and moving the gantry by hand.

Tutorial: 3D printing guides - Bed leveling

2) Second one is just more advanced version of the above. You use a dial indicator/ micrometer attached to your extruder and make sure the number on the dial is as close as possible on all edges.

Level the Print Bed on a RepRap / RepStrap 3D Printer with a Mitutoyo Dial Test Indicator

Micrometer op 3D printer

There are also digital versions of this tool.

You might need to print or make some other way a piece to be able to attach a micrometer to the extruder (there might be one for your 3D printer on Thingiverse already).

Now, remember I mentioned that you adjust the height of the edges of the print bed with a screw/nut? Well, since there is a lot of vibration during print those tend to get out of place and that might explain why you end up readjusting it over and over again. What I have found to solve this is to use a single drop of Loctite (cyanocrylate glue) in the middle of the nut/screw. A single drop will secure it in place but also not make it impossible to unscrew in the future.

3) Setting up auto-bed leveling on your 3D printer if it's modable enough:
3D printing guides: Setting up auto bed tramming leveling tilt compensation!.

Finally, if you can't get the bed leveled no matter what, it might be uneven. That's usually caused by warping. Both PCB beds and aluminum beds can get warped. With the former just sandwich a glass bed on top of it and it should be fine. Warped aluminum is pretty hard to flatten. In this extreme situation you might need to get a new aluminum sheet cut and drilled, preferably a mm thicker one which is less likely to warp under the same conditions. I think this happens when you either set the bed temp too high and/or the thickness of the aluminum bed is too small (bad or cheap 3D printer design).