I highly recommend the scotch clear 5 lbs clear tape. I tried the 10lbs black and the pack was a pain to get out. way too much effort. The clear offers more shock absorption and holds really well. Just do like Matt said and clean both surfaces and warm up the tape and surfaces with a hair dryer. once you get the pack in, it is nice to put foam on top of the pack and put the strap in. leave it over night like this. it puts pressure down on the packs and tape to help if bond better.
lastly, to assist in the pack not working its way loose, I mount something in front of the pack. I use the cap mounted to the chassis right in front of the packs. That seems to really help. I ran my car like this for months without having to replace the tape.