DYI diffuser kits/plans

Try this:
http://www.audioasylum.com/audio/tweaks/messages/1725.html

Also see John's website, although I'm not sure this article is posted there:

http://www.geocities.com/jonrisch/index2.htm

I would not go down the DIY room treatment path, unless you dont care how they look! It's not the case for everyone but consider what you need to know about room acoustics as it's complicated. There are many products out there that wont break the bank and they work. Try Sonex, RPG, Echo Busters, Tube Traps, etc.. pick up some foam panels as you get around ten 3 inch for $90 and use them around the room to tame the highs, then pick up something for the corners to help with bass, these will cost a bit more but well worth it as these companies know the right material to use, etc..
I tried the DIY and they all looked like $hit and didn't do much as to improvements. This is just my opinion. Happy Twaekng!

I second the recommendation on the Everest book above for a very readable general intro. to room acoustics. Don't let the "Master" part get in your way. It is really nicely written.

Cheers
I remain

The Master Handbook is excellent and provides lots of useful information about room problems and how to solve them. But there is almost too much information in this book. A lot of theory and specific data on many different kinds of room treatments, but not a lot of advice on how to use all this information.

I'm building my second dedicated theater right now and doing all my own room treatments. I learned a lot from the first one, from reading the above mentioned book, and from talking to a couple acoustics consultants. If you're interested, you can take a look at my website http://www.jaytorborg.com/home_theater_main.htm and click on Home Theater Design.

I agree with Jsawhitlock that making DIY acoustic treatments look nice is very difficult unless you're a competent cabinet-maker. I'm not, which is why I am burying all my acoustic treatments behind fabric walls (which are reasonably easy to make look great without a lot of experience).

Good luck.
- Jay