There are a few things to know going in. First off, though the disks say 8.5gb on the wrapper, you'll only get about 7.9gb of data on each one.
Second, if you are burning data files, you won't have to worry much about layer break placement, as that is more a problem with dvd-video. Use ImgBurn, it will work well.
And probably the most important part is choice of media. We recommend Verbatim +R DL media. The only other viable choice is Falcon DL +R, though it is not as easy to find. If you are in the US, you can find the Falcon disks at cddimensions.com. Verbatim can be bought at Office Max, Best Buy or at many shops online, like rima.com and supermediastore.com.
I like to burn DL at 4x and have had no problems using Verbatim DL at that speed. Others around here have had success burning at 6x or 8x depending in part on which speed Verbatim they are using. There are two varieties of Verbatim +R DL, the 2.4-6x, and the 8x versions.