First level the bed using a sheet of paper as described in the manual (you read it, right?). Remember to do this while the bed and hot end are both at the temperature you plan to print at. Use little adjustments of 1/8 to 1/4 turn and go round and round the four corners several times until every corner feels about right, using the paper method. Once you have that you are in the ballpark of level. Next print the version 3 bed level calibration file found here http://www.thingiverse.com/thing:2188146
Print the file then look at each corner. Ideally you want it so that each line of filament in the square it touching its neighbor so that they form one flat surface, not a bunch of disconnected bars. That means you have enough "squish". Next feel the top of the squares so that they are relatively smooth. If they feel ridged and you can see scars where the nozzle traveled across them then you are squishing too much and the nozzle is extruding more plastic than there is room for. Depending on what you find in each corner I adjust that corner up or down and print again until you have nice squares that are neither ridged nor loose bars.
I still can't get my bed level, at best I get the corners printing well but the center is too close/far. Help!
Lots of people report this problem with the original glass that comes with the printer. For many that glass is not of even thickness which causes the center to bow or sag. Try flipping the glass over and re-level to see if one side is better. Make sure to mark the sides with tape or something so you can tell which is which later on. If neither side gives good result then get yourself a new piece of glass. Get a piece cut at a local glass shop, buy a mirror tile of the right size, or cut a mirror or piece of glass to the right size. Some advocate borosilicate glass while others have success with plain mirrors so read up on it and decide what you prefer. For many people that were having levelling problems changing the glass made all the difference.
The center of my bed is sagging all the time, what can I do?
You can add support and a leveling point near the center of the bed using http://www.thingiverse.com/thing:2168244 Be aware this adds a fifth leveling point so level your corners first then in small steps use this wedge jack to support the center until it is level.
