Cutting fabric at a right angle is important for pillows, duvet covers, curtains- well, basically just about every home dec project you can imagine. The technique is also important for purses, placemats, napkin and table runner projects. The only trick to it, I think, is to have the right tools and a decent cutting surface.
Everything I list here is available at SouthStar Supply Co., a commercial sewing supply house in Nashville, TN. SouthStar has on-line or telephone ordering and their minimum order is $50. (If your order is less than $50, you will be charged a surcharge.) If you are lucky, you might have a sewing supply business in your area. That's nice, because some of the tools are bulky and/or heavy and expensive to ship.
First, I love my Salem cutting mat because it is big (70" x 48") and has a bright red grid to cut by. You can use a rotary cutter directly on it. It is not a "healable" cutting mat like you find at a retail fabric store, but you can sand it down if it gets too rough. Quite honestly, I have never sanded it because it will get rid of the markings. I just budget to buy a new one every two to three years. And if you rotate it every once in awhile, you shouldn't develop "ruts". It is a fairly thin (1/8" thick) plastic, so if your cutting table doesn't match the dimensions that the cutting mats come in, buy a bigger one and cut it down with a utility knife. (Pay attention to the description of the cutting mats- the bigger ones do not come gridded.)
Secondly, I love my work height cutting table. My husband made this one for me and he built it knowing I wanted to use the 70" x 48" cutting mat. But an old desk or sturdy table would work, though I would put it on blocks so that it is a better work height. I just noticed SouthStar has a nice folding work table that comes with a 48" x 70" Salem cutting mat. And if you don't have the space or money for any of the above, just buy a mat and use it on your floor. The mat can be stored rolled up and though it will have a little curl when you unroll it, it goes away overnight.
My other absolute must for cutting good squares is my Fairgate L-Square. Mine is the 14" x 24" English version. If you align the short edge along the selvage, you've got 24" of length you can cut at a right angle. And if you add a Fairgate straight rule like I've shown in the picture above, you can make a good cut as long as your straight rule. I have the 48", 60" and the 72" length straight rules. I use the 48" a lot, because it is easy to handle. I use the 60" occasionally if I need to cut across an entire width of 54" fabric. I almost never use the 72". If you have a drafting or art supply store in your area, they should have L-squares and metal straight rules. Also, check out a hardware store, Lowe's or Home Depot- drywallers and carpenters use them. (I don't like T-squares. They have a ridge to butt them up against the side of table, so you have to move the fabric until it is parallel with the table edge. I find it easier to move the L-square than the fabric I'm working on.)
I also use weights to hold the rulers in place, especially when I'm cutting long lengths. SouthStar sells a cast iron cloth weight that is 10" long and weighs 4 pounds. I have about six and I love them. However, this is where shipping is a downer. You just want something that is fairly compact and heavy- if you look around a hardware store, you might find something that will work. (A bag full of washers, maybe?)
Now you know how with just a few hundred dollars worth of tools, you too can cut a perfect right angle! Okay, if you are not that into your sewing or craft projects, I would recommend at a minimum an L-Square. And I personally love the metal ones because they will last forever. (Update 2/9/08: I bought an L-Square at Home Depot that is 16" x 24" for $6 and some change. They also have a 36" and a 60" metal straight rules.)