Tried to do the double slit experiment, failed. Why? I tried, and failed, to do the double-slit experiment.  I thought I had a good experiment setup, but obviously I was wrong.  Can anyone offer some insight in what I might have done wrong?  Most of this was done using some vague memory of experiments way back in High School, so there might be a better approach.
I did search through past questions.  This question was the closest to what I am shooting for, but there are significant differences and did not enlighten my particular situation.
Setup:
I used a variety of laser pointers as the light source, both red and green.  One laser pointer was home built using a red laser diode (200-250 mW) from a DVD burner going through an adjustable lens.
The slits were made by depositing soot from a candle onto a microscope slide, then "cutting" slits in the soot using razor blades.  Two blades were held together and the slits "cut" at the same time with a single swipe of the razor blades.  Initially I used old fashioned double-edged razor blades but the blades were too thin and only a single slit was made in the soot.  I switched to X-Acto blades, which made two slits approximately 0.5 to 0.8 mm apart.  I didn't measure this, I just eyeballed it.  I did both a single slit and a double slit.
I varied the distance from the laser to the slit, from inches to 20 feet.  The distance from the slide to the wall was varied, inches to 8 feet. 
Results:
I never clearly saw an interference pattern.  I saw things that were probably some kind of interference pattern, but there was no consistency.  One experiment showed a pattern of lines that was about 2 cm apart, while just changing the "quality" of the slit changed the pattern to about 0.25 cm apart (not changing the laser/slit/wall distances)-- this just seemed wrong to me and implied some fundamental experiment setup problem.
I also expected a difference between the single and double slit, but that was not the case.  The variation of patterns seen for a single slit was similar to what I saw with the double slit.
When the laser-slit distance was 20 feet, and the slit-wall distance was inches then I could clearly see an impression of the slits on the wall with no hint of an interference pattern.
Other experiment notes:
The soot was surprisingly difficult to put slit into.  If the soot was made thick enough to block all light then the edges of the slit were ragged and the width of each slit was not consistent.  If the soot was thinner then the slit was smoother and consistent, but there was some light leakage in the areas that should have been solid black.
Questions:
Is the "soot on a microscope slide" a valid method for making the slits?
Is the laser pointer with adjustable lens an appropriate light source?
Should I expect to see a diffraction pattern with only the double slit, and not with a single slit?
Did you see any other improvements I could make to this experiment?
 A: It sounds like the laser beam was too narrow to go through both slits. 
This video shows the use of hair instead of two slits. That gives you a more stable interference pattern.
I had the same problem with soot that you did, so I used a sheet of aluminum foil with two cuts made with a knife, but that still doesn't allow the nice calculation shown in that video.
A: Here is an helpful tutorial for comparison to your attempt. (I'm sure I saw this on Physics SE but I can't find it now).
http://www.youtube.com/watch?v=UANVMIajqlA
I used this method a little while ago and was able to get results.  It uses mechanical pencil lead and an over the counter laser, making it a very cheap experiment.  Let's compare the parameters of the experiments.


*

*Slit spacing distance: 0.5 mm by the manufacture of the lead / 0.5-0.8 mm for yours

*Between slits and the wall: about 1 m for video / inches to 8 feet for yours

*Slit size: about 0.5 mm with the lead / unknown for yours

*Between lasers and slit: about 1 foot for video / inches to 20 feet for yours

*laser power: 5 mW for what I did / 200-250 mW for yours


From a simple numbers perspective, there wasn't any material difference between your method and the pencil lead method, with the exception of possibly the laser power.  It's a bit beyond my knowledge whether that could be a factor or not.  Personally, I would try with a cheap 5 mW laser just to see because it's cheap.  I have always thought that a more powerful laser would give better results, for the simple reason that you'll have higher intensity, I thought it would be fun to try this with some Wicked Lasers.  One thing I have noticed is that even if you buy an extremely expensive 1 W laser, you don't really get a better M^2 value.  In other words, they're more powerful but not more coherent than the cheap ones.  My (unsubstantiated) belief is that minimizing the M^2 value would be ideal for doing the double-slit experiment.  I have no idea what the quality of your laser is, but you might want to look into this.

I also expected a difference between the single and double slit, but that was not the case. The variation of patterns seen for a single slit was similar to what I saw with the double slit.

Nope, you should see the pattern for the single slit too, although, I should say it depends.  Consider the example (from another answer) of shining a laser on a hair.  In that case, you have two "slits" in the sense that the laser can travel to the right or the left of the object.  The same thing is observed in the video I referenced here.
This makes the experiment much trickier.  It doesn't matter that you made two carefully constructed slits if there's still space for the laser to get around it.  How wide is the laser beam, and how far is the light blocked to the left and right of the two slits?  In the classic double-slit experiment, the slits are very small, such that it creates a fully non-directional wavefront propagation.  I believe this is where they get those experiments without a blotched middle area from direct shine.  We certain don't make the slits of comparable width to the wavelength and it still manages to work somehow.  I always found this amazing, and I lack full ability to explain it.
The other complication is that nature doesn't care what interference pattern you were trying to create, and it interferes in whatever way it can.  I've noticed that when holding many pencil leads (like a fence), you can occasionally see an entirely new pattern occur.  That may be inadvertently switching between one, two, three slit dynamics.
A: The idea of using soot sounds like something people did in the 19th century and that everyone then kept describing the same way as a matter of tradition. I've done this successfully by (IIRC) filling in some area of a plastic transparency sheet with a Sharpie, then scraping off the slit pattern I wanted.
Calculate the pattern you expect, so that you have some idea how big you expect the fringes to be at a given distance from the wall.
Antillar Maximus's suggestion of using a lens is reasonable, but I've found that usually all I need to do is exploit the fact that the raw beam from the laser just isn't very well collimated. Move the laser back far enough from the slits so that it covers both slits.
If you just want to see something, a hair works; to calculate what you expect, most human hairs are ~0.05 mm in diameter. Another thing that works is simply to form a single slit by holding the straight edges of two scraps of paper close together.
If you get to the point where you're trying to get quantitative data, watch out because if your beam only bathes part of the slits, then they act as a pair of slits with a smaller effective spacing.
A: Interference phenomenon are caused by the principle of superposition.This wave phenomenon seem to be violated when we use high intensity waves like shock waves , high intensity laser beam...etc .This may be the reason of your failure.
