I'll admit, I'm not really that big on "covering up" when nursing my baby - it's usually too warm for her and me, she is at that distracted age where she would just grab it and pull if I did try to use a cover, and it's just one more thing to carry around.
However, there are occasions where a coverup has been perfect. Take for example the time I had to nurse my son in the hospital waiting room when he was just a few days old. Neither of us had any idea about breastfeeding and so latching was an awkward and time consuming process (with way more exposure than I was comfortable with at the time). Or how about the times that I have ventured out to church with a baby, that isn't exactly the kind of place where I'd like to whip out a boob... Not to mention that not everyone is as easygoing as I am with breastfeeding in public, and some places even give nursing moms a hard time (that is awful, I'm glad no one has ever told me to cover up, I think they would have gotten an earful!).
I have a new black and white Udder Covers brand coverup and a borrowed pink and white Cover in Style coverup by Hooter Hiders. Both operate on the same premise, they are a rectangle of fabric with a strap attached, and a section of hem between the strap connections that has some boning to make a stiff opening when the strap is fastened around your neck. This opening can be made bigger or smaller by loosening or tightening the strap. Both nursing covers allow this to be done fairly easily using a double d-ring fastener. Once the cover is on, simply bring the baby into position, arrange the fabric around him/her, and rest assured that no one will be seeing anything that you don't want them to. I suppose if you are sitting down and someone stands up behind you, they could possibly see in from the top, but really? Who does that? (Please tell me no one does that...)
The Cover in Style has a terry fabric pocket on one corner, it's great for wiping up any milk leaks or spit up without having to wrestle with another piece of cloth. Both coverups fold/roll up quite small and so they don't take up much space in your diaper bag. I usually keep mine in the car so that if I happen to be in a place where I need it, I have it handy. They are machine washable (delicate cycle recommended), and can go in the dryer on low heat, though I usually line dry mine.
Both the nursing covers come in a great selection of colours and patterns, I would recommend a lighter colour if you think you'll use it outdoors where the sun can make it uncomfortably warm for you and baby. You can check out the selection of Udder Covers at http://www.uddercovers.com, as well as use the store locator to find a local retailer (and save yourself the $10 shipping!). Although they don't seem to make the Cover in Style anymore, you can also look at the similar Hooter Hiders and Bébé Au Lait nursing covers at http://www.bebeaulait.com. All of these brands will cost somewhere in the $30 to $40 range.