Not only is making a DL coaster expensive, but you also waste one or more hours copying and burning the DVD. I found that to minimize bad copies, there are at least three factors to consider.
1. The brand: there are a few excellent brands. Many people recommend Verbatim 2.4x DVD+R DL.
2. The write speed: I read that the slower the write speed, the better the copy. The speed 2.4x is recommended for DL.
3. The burner software: some programs burn to DL better than others and offer a variety of features, so search for the one that meets your requirements.
I'm not an expert in this field, so I'm sure there are many other considerations (hardware, source DVD, computer settings, etc.). Two excellent internet resources for DVD basics, product reviews, and free downloads are
http://videohelp.com and
http://doom9.org .