PolarAlign

You can level your mount easily within 0.1 degrees of flat.  All you need is a piece of marble or granite flooring and a ball bearing.  I use a piece of polished granite 18" square.

Grossly level your mount. Place the granite or marble on the mount.  Roll the ball bearing slowly across the granite.  If the bearing rolls absolutely straight and stops you are level in that direction.  If the bearing rolls in a curve then the direction of the curve indicates the low side.

Just for the sake of discussion...  How important is it that your mount is absolutely level? 

It isn't really.  Suppose a mount is not level by one degree.  Since the earth is nearly perfectly spherical, any tilt in the mount just means that the mount would be perfectly level if you moved it to the proper place on the earth.  In other words, the tilt of the mount just means the mount is leveled for somewhere else on the earth.  Well, how much?  It turns out that one degree shifts the apparent place on the earth by roughly 60 miles.  That statement isn't quite accurate if you aren't at the equator.  The reason is that lines of longitude aren't parallel, they all meet at the poles.  As you move from the equator toward one of the poles, one degree of tilt in the N-S direction moves the apparent level point less than the maximal 60 miles and at the pole, there'd be no displacement at all.  Anyway, you could calculate the apparent shift but you can estimate about 50 miles.

So what effect does our 1 degree tilt have?  It shifts your apparent position on the earth by 50-60 miles.  Practically it just alters the apparent meridian so things pass through the meridian at a time slightly different from your true meridian.  Basically it doesn't matter.  As long as your telescope is polar aligned, a little tilt won't affect tracking and so it just doesn't matter. 

Now back to the discussion about accurately leveling... 

Demonstration of the ball bearing accuracy: 

Place the granite/marble on a table.  The edge closest to you we'll call the back.  Use 3 shims, place one on each side near the back and one under the front in the center. Roll the bearing slowly from the front or the back. If the ball rolls in a curve, use one of the side shims to raise the low side.  When the ball no longer curves, you're level side to side.  Now roll the ball from one side to the other.  Use the front shim to correct the level front to back.  When you can slowly roll the ball bearing in any direction without a curve, you're level.

Next use a digital inclinometer and raise one side 0.1 degree.  Roll the ball bearing on the surface and you'll see a definite curve.

The size of the bearing makes little difference.   A larger ball bearing 1" in diameter is slightly more sensitive than a 5/16" bearing but either will work.  The surface of the granite or marble has to be free of grit and the bearing should be clean. 

If marble is used, the bearing will tend to follow the grain of the marble so ignore changes in direction where this happens.

This method is much more accurate than using a spirit level. You can demonstrate this by leveling the marble/granite with the bearing.  Use a digital inclinometer to raise one side 0.1 degree.  Recheck the level with the spirit level.  The bubble is still centered.

Other tips:

You can check the accuracy of your spirit level grossly by leveling a surface with the bubble.  Now rotate the level 180 degrees.  The bubble should be centered.  If it isn't, your level isn't accurate.

If you have a digital inclinometer that doesn't have a level reference (Wixie, for example), you can still use it to find level.  Use a flat surface and shims, level it as well as you can. Next zero the digital gauge on the surface.  Rotate the gauge exactly 180 degrees.  Correct half the angle.  If the gauge reads 1 degree for example, adjust the surface so the gauge reads 0.5 degrees.  Zero the gauge. Rotate back to the original position and the gauge should be very close to zero.  You can repeat which should get you dead on.

The Wixie digital inclinometer  is sold for woodworking.  People use it to set the bevel of saw blades for angle cuts.  It retails for $39.  For astronomers, it's handy to set the rough angle of the telescope tube.  When you polar align, you want the telescope tube set to the same angle as your latitude.